Let’s over-simplify this:
Twitter is a very popular site. FriendFeed is kinda’ similar. Facebook is a very popular social network site that happens to have the features of FriendFeed built in. Now Facebook owns FriendFeed. Some acquisitions are about technology, some about user base/customers and others about the talent that a company has. I would assume that almost every person with an account on FriendFeed already had an account on Facebook. Facebook can easily integrate the same technology that is present in FriendFeed (and already has for the most part). So I’m gonna’ go out on a limb and say this one likely had more to do with the talent (mostly ex-Google rock stars) that FriendFeed had amassed… But who knows, there are likely plenty of variables I am not privy to, so that’s conjecture.