Twitter has been busy making Facebook-like changes today — adding multiple photo upload and tagging, and experimenting with a profile change on iOS — but it isn’t stopping there. Some users are reporting that the retweet button inside their iOS app has become a ‘share’ button — yes, the sacred retweet button!
I think they're doing this to capitalize on the association people make with 'sharing'. Facebook, as annoying as it is, controls a HUGE portion of the social networking arena. Most site owners still gear their ad campaigns since FB most times brings in a lot of traffic, especially viral traffic.