The value is that all of my friends have it and we play online games on it. If everyone said "fuck this shit, I ain't paying for multiplayer" and went to the PS3, then I imagine MSFT would make the service free. But Bucket's right. They are in a dominant position. If you want to play online with your friends, you have a better chance of all of them having a 360 than a PS3 (especially in the US). The controller is better, the headset is packaged right with the service, etc. They made it easy. I have a PS3 and still have no idea what I can use as a headset. Any old bluetooth headset or do I need a special one? And frankly it wouldn't matter because all of my friends play multiplayer on 360.
Simply put, MSFT did several things in this gen that gave them the crown. Smart strategy, took the long road (didn't abandon things when the Wii took off), etc. I was mostly a PC gamer until the Xbox came along. Just had a ton of friends wanting to play Halo 2. We branched out over the years to include other FPSs, a little RTS where it was available, etc. 360 has been my favorite console by far, but now it's long in the tooth and I'm back playing PC titles a bit (thanks to Diablo III).
Bottom line: PSN never really had a chance; had to play catch up in the online service, etc. By the time they could match features 1:1, everyone was playing CoD on the 360 and not giving a fuck about blowing $5 per month on it. When you blow the number of hours on CoD that I've blown (especially CoD4), $5 per month is chump change.