The point I want to make is that people who don't post, or who post to filters, usually aren't doing it because they are being exclusive and don't want you to read their journal. They are, I would bet, 99% of the time, not posting publicly, or filtering you out of their posts, because they don't think you want to read about them. I don't expect all of my friends to want to read about certain things, so I filter. I'm not the only one. And I am pretty sure that most people who don't post a lot do it because they think they just don't have anything interesting to say. Not posting is not a sign of cliqueishness; it's more likely a sign of a) insecurity, or b) being really busy. I mean, on a limited time budget, which would you rather do--read fic and read your friends list, or post about how you had cornflakes for breakfast?
As for the insecurity factor, people on lj are doing their absolute best to make themselves insecure and paranoid already, what with the whole friending/defriending issue alone. It's hard enough to be here on livejournal already. When you have private or personal posts to make, you're not going to be likely to make them in front of a group of people you don't know well. That's not cliqueishness, it's caution.
It's extremely difficult to be honest or discuss your personal life in front of a large number of people. Your LJ contains a built-in audience, and that audience, for better or worse, does have audience expectations. Complaining about it and resenting it won't make the expectation disappear; but that does not obligate you to make yourself vulnerable or uncomfortable when you post. Period.
The line is different for everyone. Just because you are perfectly comfortable posting about your menstrual cycle, your dog's funeral, your horrible workday, your fanfic pet peeves, or your whatever, in public in front of a large number of readers, does not mean that your best LJ friend will be comfortable posting about any/all of those things in public. That is not a reflection on either of you.
What is a reflection on all of us, I think, is that we can't ever seem to make peace with this dichotomy: that we debate the rules of LJ etiquette, LJ-as-personal-space versus LJ-as-public-forum, friending/defriending, ad nauseum, as though there is one actual answer for all these things that make us so crazy.
There isn't an answer. But if I had to give advice to anyone who asked me how I dealt with all this crap, this is what I would say are the 3 basic guidelines for not losing your sanity because of LJ:
Accept that LJ dynamics are innately imperfect.
Know that everyone else here is just as insecure and awkward as you are.
Trust that we are all acting in the same good faith towards each other, as much as we can, as often as we can.
The trust is the most important part of this set. Seriously. It's so important, I think, to surviving the crazy fandom madness. The next time you are verging on getting upset at something you feel is directed at you or at a portion of the lj population, choose to trust in a different value instead:
Trust that someone not friending you or defriending you doesn't mean they don't like you. Trust that someone filtering you out of their posts is most likely because of their insecurity, not you. Trust that your friends are not laughing at you behind your back. Trust that everyone means well--even people who annoy you; even people who've done really shitty things to you. Because, dude, even people you hate are people who are wonderful, caring friends to someone out there. And even people you hate deserve the benefit of the doubt.
Is this an overly naive mindset? Yes and no. I'm not naive about what can happen here, because most of it, haha, has happened to me. :D I'm no more or less prone to being backstabbed, screwed over, outed, mocked, ratted on, violated, insulted, or defriended than anybody. And yeah, it always sucks.
But ultimately, I would rather choose to trust in people, and bring that trust to all my interactions on LJ, than just assume that people are being deliberately malicious/exclusive/elitist/whatever. It's not like I haven't had to practice to do this. It's a daily choice. But the more I make that choice the easier it is to make. And it's saved me more angst over petty LJ shit than you could ever guess.