I haven't felt a sense of cliquiness as such, but there are forums or threads where I tend to avoid posting on or withdraw from because they can become somewhat heated and unpleasant and if you dare to voice an opinion which differs to the main one, you can be on the receiving end of some, shall we say, vociferous opposition. It's not worth becoming embroiled in these threads as views can be pretty entrenched and you are unlikely to alter someone's opinion. I don't think that is cliquiness though, it's probably more a case of the regular posters who have differing opinions enjoy the "spats" that may arise between each other. I know, I could continue to post on these kind of threads, but really, what's the point? I'd rather spend time on the threads where your opinion is respected, even if it's different to someone else's and then move on to something else. The Politics and Religion forums tend to be places to avoid on the whole, if you don't want to risk being shot down in flames as opinions can become somewhat heated on some of the threads there!
I suppose when you first join, it might seem as if there are cliques because a lot of posters have been posting for a long time and recognise each other and exchange pleasantries and refer to things that a poster has mentioned in the past quite frequently, but the thing is, the longer you post then you, too, become recognised as a regular poster and other posters might mention you in their posts sometimes - though this is not guaranteed. I honestly don't think that that is anything personal and that you are being deliberately ignored, but just the nature of forums that have lots of posters on them, which means not everyone is always going to get a mention by other posters. Also, the more you post, the more comfortable you will feel mentioning other posters yourself.
I'm not sure, therefore, Lara, what can be done, other than keep a look out for anything deliberately nasty which may be aimed unfairly at a poster and report it for you to investigate.