I'm not sure that they are really that similar. Taking the knee doesn't really bother me as much it bothers some; they can do it or stop as far as I'm concerned. What's it achieving now - I don't know. Will it achieve anything? It's possible, I guess. It doesn't really affect anyone though.
It looks like a short protest to remind people about general conditions in society, to me.
But social media abuse comes from specific people whom it is possible to individually punish. A boycott would affect millions of people (not me, I couldn't care less what they post, apart from things like Rashford's school meals campaign). I imagine if you're 8 and your dad asks Sean Morrison to tweet you happy birthday and he does, then that's a massive deal to you. Things like that happen on social media every day.