Oh, I can just imagine how the process went at MS:
* Exec A: Social things are Good (coming to the conclusion about 10 years too late)
* Exec B: Shall we create our own Google+?
* Exec C: No, that didn't turn out that well for Google. Let's buy something existing.
* Exec B: Ok, I've Googled a list of top 10 social networking sites for sale, and ordered them by list price, but really we'll need to come up with a strategy first to make a good choice...
* Exec A: Booooring. Let's just buy the cheapest one and be done with it. What's the worst which could happen? Gimme a bonus.
To be honest, they're probably buying the users, not the product.