Twitter buys Posterous: Why?

Twitter acquires Posterous

In a surprise move, Twitter have acquired blogging site Posterous.

The question of course, is: "why?" The information from the companies is pretty vague.

Guess 1: (the view held by my Twitter community) It's a pure talent acquisition, Posterous will be shuttered, and the team will help develop Twitter

Guess 2: From Splat F:

Both posts are predictably vague about what the Posterous team will be doing at Twitter, but here's a not-so-wacky guess: Building out Twitter's photo/video/link/multimedia sharing services. These are currently rudimentary, but they're important in competition with Facebook, Tumblr, Pinterest, and other broad "sharing" services.

Guess 3: There's actually a synergy here, as Twitter start to build on top of their microblogging service with focused blogging that help feed their Discover product.

Anyone care to place a small wager?