A lot of times I skim through them because also a lot of times they are a tad too long because of the legal language. But also because they aren't hugely different - read three or four and the rest would be repeating over 90% of those.
That's for software, though, for services it varies - mostly, I tend to give them a brief look at some point but not often on signup.
However, if we're talking forums, then I definitely read them. They aren't that long, honestly (compared to other ToS, at least) and there is a lot of overlap between most of them (e.g, don't spam, don't start flame wars) but occasionally there is a difference somewhere - it might be in acceptable/unacceptable topics, or choice of language or something. If the administration has went through the trouble of pointing something out, I feel it's only fair, I give them some of my time for it.