How can we trust friendship sites?

I am a member of some of those friendship sites and I have met some great people through those sites. Some online friendships remain very superficial and don't last long, but I know from experience that it is possible to find real friends through the internet. Whether people are genuine or not that is sometimes hard to tell, and I sometimes realize that I have been wasting my time on someone who wasn't worth it, but that's just the chance I've got to take. The positive experiences that I've had mean a lot to me and I'm willing to accept the bad experiences as well.