Sorry to burst the bubble of hope, but Microsoft aren't big on co-operating with
ANYBODY.I think the reason is that they believe we should only use Microsoft products and therefore, do whatever they can to make sure they don't co-operate with the competition, whoever they may be.
I don't know much about Sony, since cross platform games are rare with them, but there have been a few that were.
Whether or not the PSN network was compatible with other networks, I cannot say.
Nintendo I know nothing about, so if someone has perspective to offer, please do so.
As an aside, how many of you know about
http://en.wikipedia.org/wiki/Steam_(software)?
Some people hate it, but for social gaming, I find it an incredibly efficient tool.
It is partially cross-platform, as you can use it on PCs and Macs equally.
I'm uncertain if it's possible to expand Steam into console territority.