Best way to face it that a friendship is over?

I have a couple friends who I don't really see/talk to much anymore. The last time I'd seen them was March, when we went out for my birthday. I always hear "we should get together" and yet when I make plans, the one is always busy and with the other its 1,000 excuses why she can't. (Her husband is an ***, but that's a different story.) I know that people have busy lives with kids & work, but is once every six months too much to ask for? I'm sick of being the one who tries to carry on these friendships. Should I face that it's probaby over? As far as the rest of them, I have a couple good friends I talk to weekly, but other than that it seems most of my friends are "fair weather" whe seldom return my emails. What do you suggest? I don't even know what a true friend is anymore.