I liken it to getting toothpaste out of a tube. Every day you take just a little more but because there's plenty left over you don't think twice about it. But as time goes on, there's less and less left until one day you realise how much effort you have to use to get that little bit extra out so you can brush your teeth that day.
It's like that with my reserves for socialising (or even communicating) with people. It's okay and exciting at first, but over time it creeps up on me how much energy it's taking. I can keep squeezing just a little more out each time, but I get to the point where it's just too hard. At that point I disappear until my reserves are replenished through A LOT of alone time.
Unfortunately in the meantime people have gotten stroppy with me for the lack of contact, or I fear they have, so often I don't then reach out to reconnect with them.
Angie, I suggest you leave them to contact you, but from time to time make an excuse to keep in contact just to let them know you still want to be their friend. This could be a one line text or email to say you were thinking of them and you'd like to catch up with them if they're so inclined. Something friendly and light but leaves it in their court to take you up on it.