I don't really think there are "cliques" on here at the moment (I don't know about in the past)....there are only 'regular posters' and 'non regular posters'. Of course its easier to think of something to say to somebody if they post a lot, because you know more about them, but I think this is probably the one kind of forum where there wouldn't be cliques.
As for the trust thing, I get you. The absence of physical interaction DOES make it harder to tell who is genuine and who isn't. Personally I like to give everybody the benefit of the doubt because, well, it feels nicer that way. Though I know that can also be dangerous and that's why apart from my eye and my hand I won't post a picture of myself here on the forum, nor state where I live. For one thing, hundreds of 'guests' are trawling through here all the time. Guests a bit much like I was, when I found the site, before I ever considered that I may need to be "one of those people that takes part in internet forums".
I would gladly say I have found "friends" here, just a different type of friend to those I have in real life. Essentially, we're all here for the same reason - to find support for our social anxiety, and it's nice to be able to help one another. I think the key is to just not become too reliant on these people, and give away too many details, until you're more certain that they can be trusted, and how long that takes depends entirely on how long you've been chatting and the depth of your discussions.