Instead of "ditching friends", my approach to flakey people has been to mass invite people. I've ended up with a core of about 4-5 very reliable folks for any type of event I'd invite them to, and the rest can go or be louts and stay at home if they want (ok, only a few folks do this, the rest just overbook their time).

EDIT: A note: There are a couple folks that I've effectively ditched. I no longer take their schedules into consideration because they were such a PITA. Folks: If people invite you to things every week because you tell them you want to be invited, and you never show, don't feel hurt when they stop considering your schedule. Pouty friends are annoying friends. Annoying friends cease to be friends.