Here's one theory. This is another escalation of the battle between Twitter and Facebook's news feed. Both are life-streaming/microblogging tools that capture the essence of real-time thought -- and data.
But where Twitter falls short is in the conversation. Threaded conversations just do not happen on Twitter. They are fragmented conversations that are most often seen to the public when you are following both sides of, and are parties to, the conversation.
On FriendFeed and Facebook on the other hand, replies, and therefore conversations, are threaded, creating streams of consciousness that are actually organized. Back when my Twitter and Facebook accounts were linked, I'd often get more responses to Tweets on Facebook than on Twitter, likely because of the way the news deed handles status updates -- keeping reactions to those updates conveniently listed below each one. FriendFeed handles updates in the same way.
So at the end of the day, while Facebook may be acquiring FriendFeed for the talent behind it (the folks behind FriendFeed were also been behind internet juggernauts Gmail and Google Maps), the rationale is the same. There is a battle for the real-time data feed. And as of right now it's a two-horse race (Facebook vs. Twitter).