IIN that fat people should get compliments on social media?

Honesty doesn't help people change: You're an asshole; I have zero faith that comment will make you change.
If you mentioned patronising compliments, or people actively promoting weight gain, sure maybe there would be something to talk about. But you're just talking about people getting compliments on social media, who happen to be overweight. That's normal, that's called having friends and being nice to people.