Indeed, it can. You may want to think twice before posting how horrible your job is or that your boss lacks basic managerial skills. A growing number of companies are "Googling" employees to see if they are thinking of quitting, going to the movies when they call in sick, or talking to competitors. Currently, almost half of employers conduct social media searches on applicants or current employees.
Nate Fulmer was fired because his Christian boss, a conservative Christina, didn't like his comments on organized religion. Stacy Snyder lost her job because her boss found a picture of her drinking a beer. (Don't ask me why.) But the worst thing you can do is talk about your boss. Dan Leone worked at the ticket gate for the Philadelphia Eagles for 6 years. A lifelong fan, Leone was upset when the team let the star defensive back go. On his Facebook page, he posted that Eagles management "R retarded". The Eagles management heard about it and Leone was fired.
So don't assume that since social media sites have passwords and privacy settings you're safe to rant freely. It isn't that difficult for your boss to find your posts or get your password. Sadly, most victims caught in this situation have no legal rights. The first amendment does not apply to corporations and only four states have passed laws in this area.
Until we have better laws, you'd be wise to remember that everything you say on the internet can be found and your employer might be reading it.