i don't think i've had very many friends except this kind.
I think usually what's happened for me goes like this:
I generally have a low number of friends, so these friends assume that i'll put up with more crap from them and will work harder to preserve the relationship than someone who has, well, a lot of other friends to choose from. Thus they put little into their relationship with me, forgetting to invite me places or forgetting to call me back, whether it's intentional or not (i like to hope it's not). They put a greater amount of effort into their relationship with their more social friend(s), trying to make themselves stand out in the crowd (of that person's friends), and the whole thing can sometimes have a sort of competitive feel to it. They get addicted to trying to win this person over, even if this person's attention is not worth competing for.
A few months pass, and gradually they aren't in contact with you any more. Two things generally result: they realize they haven't talked to you in a long time, feel embarrassed about it, and avoid you to avoid the awkwardness; or, they realize they haven't talked to you in a while, assume you were the one that initiated the break, and wonder stupidly what they did to piss you off so much.
each experience i've had has different surrounding circumstances, but this is one of the general patterns i've noticed, unfortunately. the only advice is to keep trying until you find a friend that gets you. or, at the very least, makes an effort to get you. i'm still looking for that good friend myself.