Ever since the end of the private beta, every time I post an answer it puts this obnoxious "share your answer on Twitter, Facebook, LinkedIn" bar at the bottom of it. I don't see that when I post answers on other SE sites. Why is it here? Where did it come from, and is there any way to turn it off?
You're the only one seeing that bar on your own content. It's to remind you that, as long as you went through the trouble of providing a particularly interesting, high-quality post, why not let people know about it? That's the only way users will continue to find this site and, hopefully, become contributors of their own.
One of the toughest challenges facing this site will be reaching a critical mass of users. Without a lot of users asking and answering interesting questions, this site will not survive. So with every site we launch, the more-engaged community will inevitably start asking "how do we attract more users?"
So we tell them.
The largest spikes in traffic every site experiences, without fail, are from those Tweets and other links to a particularly intriguing post. If you want to help this site grow, use those social bookmarks. Otherwise, just ignore them; Hopefully other people will share.