Users are a pretty weak moat. We know this because if they were a good one, Reddit wouldn't exist. We'd all still be on Myspace or Digg or wherever. Apple, Microsoft, Google, those guys have moats. Leaving their ecosystems is HARD. Leaving Myspace, Digg, Facebook or Reddit just because there are less users somewhere else isn't really that big of a loss. Once people have a good alternative they do it pretty quickly. The history of rapid turnover in social media giants compared to other tech sectors is the proof.

If you wanted to be edgy I think you could even argue users are an anti-moat for social media because they age, become uncool, and then the next generation doesn't want to hang out in your uncool boomer space and goes somewhere else. This is FB's current predicament.