I don't really understand why this bothers you, first of all. It's having zero material impact on your life and you're not obliged to follow anyone on social media, or to use it at all.
My dad was never on social media. But I always used to share some pictures of him on Father's Day. I did that because I loved him, I was proud to be his daughter, and I wanted to share that with people and to celebrate lovely dads in general. Among my social media followers are my mum, siblings, various relatives, and also a lot of friends of mine who remembered my dad from when we were kids, and who were fond of him and always asked after him.
A lot of my friends did/do the same with their own dads and I always like seeing the photos.
Obviously everyone also tells their dad all this stuff in person.
To me, it is pleasant and enjoyable to see people celebrating an objectively nice thing - in this case their relationship with their dad, but it could be their kids starting school or graduating, their wedding anniversary, Mother's Day, whatever.
I don't think that's 'performative drivel'. I think it's people talking about a nice thing with their friends.
Fortunately my friends aren't the sort of people who are bitter and resentful about this type of thing.