Really perplexing. While I was at Yahoo! they bet the farm (so to speak) on Facebook integration across their sites. Of course it was more or less a flop. I guess the next best thing is litigation?
If you can't beat them, join them. If joining them doesn't work out, sue them.
I'm worried, can I click on the "like" button underneath the document without infringing on a patent?
tl;dr
Yahoo is a social network. Facebook did not get there first.