[cableshaft]

It's not quite a social network, but Skype got quite a bit of use at my previous company. If there was a topic that people wanted to share links or ideas or discuss things about, we created a new Skype group with everyone pertinent in it. We had serious and less serious Skype groups too.

So we'd have one for the Project X team discussion, and another for Project Y team discussion, one that included the QA team if we needed to discuss bugs with them, and we'd make a temporary one if we just needed to discuss prep for an upcoming meeting, etc. And then we had the group that wanted to share meme pics and youtube links, and the group that wanted to share links to metal music, and another to discuss anime, etc.

You could also put in filters so it only notifies you if certain words appear in the chat, like your name or some feature you're in charge of. It was simple, dynamic, and more effective than any other company I've worked for before or since.