I largely agree with you, which is why I decided to participate in this post. I felt you were just losing the focus a bit from the frustration you came in with and from some of the people arguing with you here.
Like you said, I'd love to see more community building and the growth of actual movements here, but I don't know if that can be done on a generalist platform. I've got 6000+ subscribers here, and like I said, I'd say maybe a dozen of those users have actually been moved to do something. I don't begrudge the ones that are just "cute pic!" and move on with their day. If this was a comment section on www.progressiveleftycauses.com, I could be mad, but that's not what this is. I think it's important to maintain realistic expectations for a social platform. Everybody can easily jump in with a joke or a complaint, but getting strangers to do stuff is really hard. Hell, I can't even get most people to actually reply with "Great pic!" It just is what it is.
There's still nothing stopping you or I from pushing out agendas here. But that also goes for the jokers and the party crashers. I do agree that this platform may lose the inertia to become a practical haven for progressive causes as its major purpose, but it can always still support some of that.