My friends and I all finished last gen with Xbox's, and this generation everyone but myself picked up a 360.
Things have changed though, recently my friends have been buying PS3's. One had one for years, but never played it because he loved his 360 so much. He games a lot, and finally decided it was time to play some games for it. Unfortunately it broke down, now he hates the PS3 more than he did in the past.
Other than that one friend felt like he was being nickle and dimed by Microsoft so he decided he had enough and picked up a PS3 a bit before the launch of BF:Bad Company 2. This is the second friend of mine who switched because of all feeling cheated. He loves it, and I can never see him go back.
Another one of my friends, who also started out with just an Xbox (because of Halo 3), picked up a PS3 last fall/winter after he saw me play inFamous over the summer. Unfortunately for me (I only have a PS3), he has mainly been buying his multiplatform games for the 360. He has both, and even though his 360 broke a little after getting his PS3, he got it repaired. I cannot say he would do the same if his PS3 broke.
My other friend saw his 360 break for the third time, but this time it was no longer under warranty. He got someone to fix it for free, but it didn't really work. He eventually just picked up a PS3, and he has been raving about it. The release of Reach tempted him to get a 360 again, but he doesn't believe the cost to be worth it. What's nice about this situation is that it drove another one of my friends to pick up a PS3.
While this other friend still has a 360, but he really wanted to play MGS4 and has had no shortage of fun games to play. What is funny is that he has both console versions of RDR, just so that he can play online with my friends who have all converted to the PS3. The best part about this friend getting a PS3 is the fact that he called me an idiot for choosing to get a PS3 back in 2007.
Now, three years later, for the most part my friends have picked up a PS3.
360 PS3 Wii
2006 8 0 0
2007 8 1(Me) 2
2008 7 2 4
2009 6 5 3
2010 5 7(was 8) 1
Obviously this sample isn't large enough to draw any big-picture conclusions. It was just interesting for me to actually plot it all out.
Of my three friends who completely left the 360 it was because:
Extra Costs: XBL, battery packs/batteries, harddrive ect. x2
Hardware Failure: x1
My friend who didn't continue to play the PS3 stopped because of hardware failure.
As for the Wii, we all just stopped playing because of the games. My one friend who still uses theirs, is only for when other people are over. I still have mine, but nobody wants to play SSBB with me so it doesn't get used.