I signed on and paid for a year on classmates.com around the time of my ___th reunion; so I was more curious at the time. I don't think it cost very much, and it is a very good resource.
You just have to hope that the person you might be looking for is on there as well, or at least start a chain, of getting in touch with someone who could give you a lead. But I think for a year, to check it out, it's worth it. Especially when your school itself doesn't have any kind of alumni database online.
I did track down a couple of old school chums I'd lost touch with; after a few emails, with one I realised why, with the other, it's nice to email every once in a while now.