I don't understand all the hate on this thread. Use Yammer at my current startup; used Yammer before at bigger companies.

Terrific product. There's a lot of features that I'd love to have, but this is a much-need space.

A social-network for work. i.e. a social network that people will actually pay to use.

Who has perfected that? $142m is really the start here. Looking forward to their future.