While I certainly enjoy single player titles (if I was forced to compile a Top 10 favorite games list, I think every single one would be single player exclusive), I have to ask: What is the point of this sending this message?
If it's to let publishers know that we want single player games, I'd argue that not only is that unnecessary, but this is an ineffective way of communicating that. We've been blessed with a multitude of really good single player experiences this year; Breath of the Wild, Resident Evil 7, Metroid: Samus Returns, Nioh, Nier: Automata, Horizon: Zero Dawn, and Fire Emblem Warriors just to name a few; and that's before the holiday season even gets underway. The critical acclaim and/or sales success of single player titles has pretty clearly communicated to numerous developers that consumers still want single player titles. I don't think that's something we need to communicate, and even if we did, we'd be far more effective in communicating by, well, simply buying the games. What developers do you think we'll convince with a hashtag campaign that haven't already been convinced by sales numbers?
If it's to let other video game fans know that we enjoy single player games: I think we already do a pretty good job of that already.