Social Status Needs

Services that hook users typically give them an identity, whether it's a real name or just a username. With that identity, users can gradually invest—time, energy, money—making it "valuable" through systems like reputation, levels, and points. Once vanity is satisfied, they keep coming back to nurture their virtual self, and retention climbs.
In anonymous apps, users have no identity to invest in, so there's no reason to return once the novelty fades.