Since you aren't generally that active with them, it was probably just an oversight. Might try something like this: Make a positive comment about their pictures/outing on Facebook. Maybe . . ."Looks like a great time had by all. Let me know next time you get together. I'd love to catch up with you." I really don't think that kind of comment would be taken as offensive or rude. I have various groups of old friends and sometimes the get togethers are rather quickly put together/impromptu/informal, and it isn't a matter of trying to exclude anyone as much as just the nature of the informality or quickness of it all.