they were fads. Variations/customizations in one way or another of a social networking. The social wave has passed and the crystallization in its wake has left only options significantly differentiated in parameter space from each other - FB, Twitter, Snapchat, WhatsApp.
I don't know that the social wave has passed, but it does seem like single-serving social apps are on the way out unless they have a difficult-to-replicate twist to them (like Secret or whisper).
