This question was asked by the Sarah Walker Fangirls and is the subject of some online controversy (there is a video on youtube). They asked Zach and he was clearly thrown off (I consider it to be a personal question), but answered jokingly that he only follows people who write jokes on twitter, and that was why he was going to unfollow Adam Baldwin. He then apparently did “unfollow” Adam and Yvonne unfollowed him. I think it has nothing to do with any “bad relationships” among the cast members, but more with the fact that they might have felt a little too much intrusion on the question of whoever they choose to follow on twitter.
I think they are a great cast, and considering that they work together everyday, there is really no need for following, specially if some fans are apparently going to be monitoring when one of them tweets something to the other or retweets something the others wrote.
I think the issue of who they decide to follow shouldn’t be even discussed. I think it crosses the boundaries of their private life. Let them choose.