Caron, I've been thinking about this some more. Totally guessing, but I imagine the youngest of the 4 girls is now in fulltime school and that's why you and DH are no longer needed to babysit so often. If it's any comfort, please realize that your time with them was bound to lessen, anyhow, for this reason alone.
But it also occurs to me that DS and DIL may feel they left you people with the girls too often, over the years, and that, as a result, you became too much on the level of "parents." So maybe they're trying to rectify that now by cutting back your role in the girls' lives/cutting back on visits. To you two, this looks like an "overnight" change, but to them it may be a matter of fixing a problem that has been bothering them - or, at least, bothering DIL - for a long time. That doesn't explain the gift issue, excuse the cruel FB comment or change the fact that this is hurtful for you. But perhaps it explains a little bit where this is coming from.
I get your worrying that DIL "doesn't want (you) around." But on the face of things, it seems as if she's ok with you and DH coming to birthday parties. It's probably not so much a matter of trying to keep you away as trying to set new and stricter boundaries. So DIL may be fine with your appearing at a birthday, as long as you stay within the new boundaries and don't try to be included in events that doesn't cover. If you find it's too tense when you attend the next birthday party, then by all means, rethink this. But if not - or if DIL avoids you to minimize the tension - then no reason, IMO, not to continue going and seeing/celebrating with your GDs. Best of luck!