It’s an algorithm, and they’re always changing it. They’re open about it being an algorithm instead of random, but they aren’t open about what the algorithm IS. Which is understandable, then we could break it for infinite sats.
Basically, users have noticed it’s based on participation, referring friends, having a wallet attached (more specifically depositing sats into the fountain wallet), commenting, liking comments, and just general participation. I received a reply from a content creator, and then the next day I got 1500 sats in one hour.