It seems as if Valve has rolled out a new gamerscore-esque ratings system overnight, as many Steam users woke up to find "levels" next to their profiles. Evidently the update lets players earn XP, as they earn badges for community tasks. Tasks include things like "assign an avatar" or "link your social media accounts" earn badges, which will in turn, earn you levels.
All of this will probably remind you of the Steam seasonal sales, which have scavenger hunts and the like with similar goals. If you look in your profile page you can check out your badges. Like all meta-games, I'm not too worried about this. People seem to get up in arms about achievements and the like, but I just view them as a fun way to augment the experience -- as is always the case, they're completely optional.
Look at your profiles [Reddit]