The thing with lots of people being around, is that often, as a lone parent, you can kind of get overlooked.
If you have a partner/DH whatever, it can sometimes matter less if you don't see people that often, as you have instant company at home. So if you and your kids go out for the day, (assuming it's not special family time) it probably wouldn't occur to you to invite your lone parent mate and their kids.
Not because you don't care, but because it just doesn't cross your mind. When most people make those sort of plans, they don't always do a mental scan to see who else they can invite - why would they really? My friends are lovely people who love me and my kids, and are happy to spend time with me and the kids, but they don't always think of us iyswim.
I have a lovely friend who I see pretty regularly, but it tends to be when her DH is working as that is when she is on her own with the kids, and wants/needs some company. I do understand that, but it doesn't seem to occur to her that for single parents, it's like that all the bloody time!
Would you be able to ring round a few good friends and explain how you feel? Say that you and DS are going stir crazy and can you meet up for the day? (although I do realise that after the 3rd time someone says "sorry, we're doing family stuff" you lose the impetus to phone)