Cliquishness is part of human nature. And it's a part of human nature that's saliently on display in journalism, filled as the sector is with people who are verbally adroit, at least somewhat socially inclined and in more than a few cases less independent-minded than they think they are. As someone who's spent his career mostly in the distinct but occasionally overlapping fields of political journalism, financial journalism and science journalism, I can vouch that cliques form readily in all three. (In fact, financial journalism being the most diffuse, with a broad range of publications and audiences, may be the least susceptible of the three.) And while cliquishness can encourage intellectual laziness, or just a tendency to retweet your friends' not particularly well-considered tweets, it can also foster useful collaborations, and in any event it is, as I mentioned, part of human nature and so not going away anytime soon.
All of which is by way of recommending Reihan Salam's Daily Beast post on JournoList and "What Liberal Bloggers Can Teach the Right."