You don't. We live in the wonderful age of keyboard predictions where you type. Comm and community and communities both pop up as options. You'd have to take extra steps to get it any shorter.
Unless, like anyone with sense, you just type c/ or c/s and have done with it :)
I like "communities" but the problem is that in a broader context, no one will know what you are talking about. "I saw it in the memes community" sounds too vague. Saying "I saw it on the memes subreddit" is perfectly clear. Because of this, I think "sublemmy" is a better, more descriptive term.
That’s inherent to the fediverse. You have to specify which instance you’re referring to. The wording between “community” or “sublemmy” isn’t actually helpful when you have to provide the instance domain anyway.