I'm not sure we can handle another world-changing social network, but they keep coming anyway. And they keep going. But not all of them.
So why do some stay, and others go?
The reasons, and many there are, aren't quite what they used to be. The reasons Google+ is becoming more of a ghost town everyday and Path has lost its buzz are way different from the fates of MySpace and Friendster. Inversely, the reasons Instagram and Pinterest as of late have exploded are different from the reasons Facebook and Twitter dominate social networking.
To start, where the big two have succeeded, and others have failed, is creating platforms that satisfy the needs of a broad user base. Sure, it's a little soon to say they're here to stay, but it's hard to argue that Facebook hasn't turned itself into an effective tool for third parties to leverage. Twitter's working in that direction, in many ways (in tweet media is a small step of a larger shift). But both have opened themselves up enough to convince others that working with them is better than working against them.
Part of this convincing also has to do with stability and consistency. Friendster was never stable, and therefore died off. The MySpace experience was too varied, too messy. The more Twitter has addressed its stability problems, the bigger it has grown. A Facebook outage is rarer than a solar eclipse, and consistency is the name of their game (you know how to navigate my timeline just as well as I know how to get around yours).
Point is, they're at the point where they're satisfying our needs before we know we have them, and doing things right that we don't even think about. Google+ would like to satisfy these needs, but they're playing catch-up and actually proving that scratch is a poor place at which to start. Path might work with the big guys, but they don't add a significant value.
So why bother with new networks?
Because the most effective new networks now recognize that they shouldn't go head-to-head with the big guys. Instead, they're working within their rules to build their own followings. There's a new game being played: social networks built on social networks (dare I call it Social 3.0?!).
Instagram used Facebook and Twitter to grow to the incredible 40-million user base (an incredible 10 million boost in the days following the acquisition announcement) they now have by making the core of their product, the photos themselves, available outside of their app. Their only web presence is only accessible through links found on social networks. Pinterest's explosive growth was spurred by its integration into Facebook timelines.
These guys have identified that the most active Internet consumers live on Facebook and Twitter. By integrating them into their own, new, networks, they've ensured that their user base is comprised of the type of people that share a lot and participate actively. Assuming the idea appeals to a wide enough audience, is almost a guaranteed formula for meaningful growth.
What may have convinced Facebook to snatch up Instagram was probably the data. They know that the user bases overlap. Not only that, but it's the same actively-sharing, data-providing audience they're familiar with. What they wanted was the data provided by all Instagram photos, not just the ones posted to their platform. Robert Scoble recently noted that Instagram has an incredible amount of location-based data, which, when combined with the data Facebook already has, could be immensely powerful. Especially when it comes to matching imagery with specific places and popularity (even if location isn't specified in an Instagram photo, they're taken with Android and Apple devices, all of which have GPS and naturally tag locations when the shots are taken).
The moral of the story, is that to be effective at social networking, and to become a player on every platform, you have to play nice. Whether you're an app or a content provider, working in the ecosystem and not against it is a surefire means of success. The basics are already there, so if you're going to improve things (and be valuable to users) you have to expand on a specific point, like photography, within the current architecture.
Notably, I'm forgetting Tumblr. But, I'm not. That's being saved for another day.