Personally, don't have a Facebook. Just sayin', Gabe.
By increasing people's reliance on Facebook, Valve is just increasing Facebook's lifespan. Like Myspace, Facebook will one day die, as people (oh, how fickle they are) will move on to something more new and more innovative. However, Facebook is already surely to last longer than Myspace, since almost everything on the internet is somehow connected to it. Unfortunately, this will just create an inconsistency down the line where this "single-player plus" will be connected to an obsolete system no one can take advantage of; at this point, it will just be "single-player" again.
Steam has its own networking thing as it is. It has the instant messenger and a place to comment on your friends. Instead of riding the coattails of a social network, Valve needs to work on increasing the pervasiveness of their own system. And surely as hell, Valve is probably the only company with the ability to succeed with this. The only problem here is that they will then be a game developer, a digital distributor, and a significantly large social network. Good for them, but bad for competition.