NEW YORK — Facebook's stock fell to $19 for the first time on Friday, losing half its market value since the company's initial public offering in May.

The stock dipped 87 cents, or 4 percent, to briefly hit $19, just minutes before it closed the trading day at $19.05. Facebook's shares ended the week down nearly 13 percent.

Facebook hit the $19 milestone a day after the expiration of a lock-up period that had previously prevented some early investors and insiders from selling their shares. Stakeholders who owned a combined 271 million Facebook shares before Thursday can now sell their holdings.

A breakdown of just how many major Facebook Inc. shareholders sold their stock this week won't be available until next week at the earliest, when sellers must disclose such transactions.

Facebook's stock has struggled since the company's mid-May IPO. It closed its first day of trading barely above its initial offering price of $38. It has been below that level since.

The stock has been down on 38 trading days, up on 25 days and unchanged on one since its initial public offering.

Investors have been concerned about the social network's ability to increase revenue and make money from its growing mobile audience. Many analysts, however, hold positive opinions of the company's long-term prospects.

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  • Show Yourself Breastfeeding

    This mommy controversy has long plagued Facebook, as the company states there can be no nudity in its <a href="https://www.facebook.com/legal/terms" target="_hplink">terms of service</a>. But parents argue there's a line between "inappropriate" and "legitimate" images. <a href="http://www.huffingtonpost.com/2012/02/08/facebook-nurse-in-60-brea_n_1263532.html" target="_hplink">Emma Kwasnica</a> is a breastfeeding advocate who often posts pictures of herself nursing, and as a result, her account has been suspended five times. Kwasnica and other mothers even protested the issue at Facebook headquarters during National Breastfeeding Week.

  • 'Pretend' To Be The Zuck

    Apparently there can be only one Mark Zuckerberg on Facebook. <a href="http://www.pcmag.com/article2/0,2817,2385254,00.asp" target="_hplink">According to PC Mag</a>, in 2011 Mark S. Zuckerberg was kicked off Facebook because of "identity fraud." But this lawyer from Indiana had other Facebook woes before his restricted account: Because of the similarity of his name to the ever-fascinating CEO, S. Zuckerberg was receiving nearly 500 friend request a day. Eventually, after making a few headlines, Facebook apologized and the lawyer regained access to his account.

  • Share Names With A Celeb

    Selena Gomez was recently banned from Facebook. But it wasn't the Disney superstar who's been prohibited from uploading her latest pics; it was just a regular girl, <a href="http://www.tmz.com/2012/08/05/selena-gomez-banned-facebook/" target="_hplink">TMZ reported</a>. One day Selena Miranda Gomez from New Mexico attempted to access her Facebook account and found she was unable to log in because the social networking site believed she was impersonating the actress, which is against the company's policy. At the time of publication, it was not clear whether Gomez's account had been reactivated.

  • Set Up A Profile Under Your Famous Pseudonym

    Salman Rushdie, who penned titles like <em>Midnight's Children</em> and <em>The Satanic Verses, </em> had his <a href="http://www.huffingtonpost.com/2011/11/14/salman-rushdie-facebook_n_1092828.html" target="_hplink">Facebook account suspended</a> in 2011 because of what Facebook perceived to be a name discrepancy. While Rushdie's first name is Ahmed, the world knows him by his middle name, Salman. The social network told the author that he wold have to use his first name on his profile. "Dear #Facebook, forcing me to change my FB name from Salman to Ahmed Rushdie is like forcing J. Edgar to become John Hoover," Rushdie sounded off on <a href="https://twitter.com/SalmanRushdie/status/136136147398168576" target="_hplink">his twitter account </a>following the incident. Facebook later restored <a href="https://www.facebook.com/rushdie" target="_hplink">his profile</a>.

  • Coordinate Hack Attacks

    <a href="http://www.youtube.com/watch?v=kZNDV4hGUGw" target="_hplink">Operation Payback</a> was a plot from the infamous hacker group Anonymous to take down Visa's website after the credit card company cut off donations to Wikileaks. Hackers gathered on both Facebook and Twitter to plan and promote an attack, causing their accounts to be suspended on the social networking sites, <a href="http://www.zdnet.com/news/wikileaks-hackers-attack-visa-get-banned-by-facebook-twitter/490442" target="_hplink">according to ZDNet. </a>

  • Take Odd Pics Of Your Kids

    <a href="http://www.huffingtonpost.com/2012/07/27/lauren-ferrari-banned-facebook-breastfeeding_n_1709928.html" target="_hplink">Lauren Ferrari was banned from Facebook</a> for seven days after she posted a photo of her 5-year-old pretending to nurse her younger sibling. While Ferrari didn't think much of the image when she uploaded it, both Facebook and the police found the photo to be problematic. The Seattle Police Department said her actions showed "poor parenting," which sparked an online controversy about what should and should not be put online.

  • Spam Your 'Friends'

    Adam Guerbuez was fined $873 million after sending out more than 4 million spam messages about penis-enlargements, porn and marijuana, <a href="http://www.pcworld.com/businesscenter/article/207046/facebook_spammer_tries_to_cash_in_on_873_million_fine.html" target="_hplink">according to PC World</a>. This behavior got Guerbuez kicked off of Facebook and caused him to file for bankruptcy in 2010.

  • Pretend You're Over 13 When You're Not

    <a href="http://www.huffingtonpost.com/2011/03/23/facebook-underage-users_n_839437.html" target="_hplink">In 2011 Facebook's chief privacy adviser</a> said that an average of 20,000 underage Facebook user accounts are shut down each day. The social networking site has a strict policy stating that only those over the age of 13 are allowed to maintain a personal profile.

  • Add Script Or Code To The Site

    <em>"Hello, Our systems indicate that you've been highly active on Facebook lately and viewing pages at a quick enough rate that we suspect you may be running an automated script."</em> How would you like to get that email from Facebook? That's exactly what happen to tech-blogger <a href="http://scobleizer.com/2008/01/03/ive-been-kicked-off-of-facebook/" target="_hplink">Robert Scoble</a>. Apparently he had added an address book importer to his Facebook account, but any additional script whatsoever just doesn't fly with this social media site. His account was restored after he "<a href="http://scobleizer.com/2008/01/03/ive-been-kicked-off-of-facebook/" target="_hplink">made a public stink</a>" about the ordeal online.

  • 10 Spoof Look Back Videos

    10 Spoof Look Back Videos

  • Vladimir Putin

  • Prince Harry

  • Walter White

  • Darth Vader

  • Rob Ford

  • Kanye West

  • Facebook's Own Look Back

  • Stephen Parkhurst: An Honest Look Back

  • Patty's Way: 'We Know Where You Live'

  • Trip and Tylor: An Honest Facebook Movie