While I agree that is some worthy snark and I am chuckling, I would point out that for at least the forseeable future it is very likely to be the other way around, due to the fact that young people have worse credit scores (if for no other reason they've had no time to build them up) and Facebook will correlate with younger people for a while yet. (Certainly older people are on there; I said "correlate", not "be solely occupied by".)

So would you join Facebook, etc., if and when being on it is more likely to raise your credit rating than lower it?

There isn't much I could do to raise my current credit score without becoming fabulously wealthy, so, no. A hypothetical other me might consider it, but... remember, it's not just "being on it" or "not being on it", it's who you are friends with. The real problem isn't the direct effect on your credit score, the real problem is that if you're even thinking about your credit score when considering who to be friends with it distorts the entire experience, possibly to the point of ruining it. The net effect of everyone thinking about that is going to change the dynamics in a generally unpleasant way.

If you don't have an online presence, it must mean you have something to hide...