I think that there are certain rules about making friends that some people miss out on learning when they are younger for whatever reason (I certainly did).
I would suggest keeping facebook - it's the only place online where what you see is what you get. If you don't like someone's comments, use the hide option or just de-friend them. Do keep posting but remove people that you don't want to bother with. Social networking is great for getting to the root of friendships - you can see what people are up to but also you are selling yourself in a way, the more you post the more people see who you really are.
I wonder if you are a bit of a chameleon with friends, trying to please them. This means that you are never your true self and will lead you into false friendship.
Focus on the people that you really like or think you can get on with. Book coffees in advance, rather than on the spur of the moment.
Also accept the fact that some people are good in different roles if you like - I have one very old friend that I hardly ever see but know she's always there. We had a big bustup in the past, but got past it. Never dismiss people you've had a bustup with, sometime those are the people that actually really care about you!
I have kids school friends, rapidly diminishing in number as they all get very busy. Occasionally useful for a trip out somewhere but I don't hang onto them for general company. We are friends enough to whinge and complain to each other but don't depend on each other in crisis or whatever.
I also have shared interest friends, in clubs or groups, again, I don't expect too much from these people as there's a lot of falseness and power games.
One trick I have learned is that if you're in a group and feel left out, find the other person in the group that also feels left out. There is always another, and it's not worth the effort to try to ingratiate yourself to people who want to be exclusive.
Online is very similar to offline I find, with the added complication of anonymity (like on here) but a good source of light-hearted relief from loneliness.
Also, enjoy your children's company - they need friends too.