I'm thinking out loud with this one, but memes tend to be a comedic way to relate to the topic in my eyes. Comedy tends to be lower friction, so it stands to reason memes would be a good shibboleth for an online community.

Now, in the modern age, you have 'creators' who make the memes and are the original posters, but you also have easy mechanisms for sharing already-created content. Sharing content is _SOMEWHAT_ akin to the original meme creation since you're putting skin in the game and standing behind the meme's sentiment. Akin to wearing liberty spikes to show you're part of a punk community. You didn't create 'liberty spikes,' you're just representing them. In using the sharing button, I think average people can interact online in a much different way than it used to be.