I have some friends who have that problem. I am thinking of two in particular where it's like both of them just completely drop off the face of the earth for months at a time.

So, speaking as someone who is friends with people who do this, here are a few things that I think make it easier on the friend:

1. Avoid making promises or plans that you probably won't keep. If you know you are probably not going to want to leave the house, it's so much better to just say that up front and not cancel at the last minute.

2. Be careful about overwhelming your friends when you feel good. Sometimes I feel like one of my friends is trying to make up for lost time when she is well. I don't hear from her for months, and then she calls me every day.

3. Don't take it personally if people can't make time for you immediately.

4. Let people know why you aren't calling/hanging out. I'd much rather know that someone is not getting back to me because they just don't feel like dealing with people than worrying that I did something that made them mad.

I do think it's possible to keep relationships up. A big part may be just to get over the fear/worries that you have driven people away by being depressed. Sure, some people may not want to resume a friendship, but a lot will be happy to hear from you and to know that you are feeling better.