I think a lot of it is point number two. Microsoft did a great job of making players feel invested in their gamer tag right and online ecosystem from launch, so even if you didn't know which version was better, you'd probably choose the 360 version of a multiplat.
This is definitely a major factor in my thinking, most of my friends, and I've heard it a lot here from Boritos as well. But I think MS didn't really realize how critical the social aspects they implemented would become. They acted on keeping them vital quickly, once they realized how much people valued them, but I think they were initially surprised at how valued they were.
I forgot to mention friends lists. Once you have friends you like to play with online it's also more difficult to switch to a new system.
Let alone the fact that it's hard to play with friends on PS3 even if you have them registered there. My PS3 gaming friends frequently tell stories about how messed up their gaming sessions can get, between chat malfunctions, server problems, not being able to find each other... it all sounds like PC internet gaming from 10 years ago, but the 360 has all that stuff working as part of their overhead, not left to each developer to implement separately. And MS' multiplayer features have continued to improve and remain integral and consistent, while Sony's have just struggled to play catchup.
