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.