Why are all the decentralized social networks very Twitter-like? 

Because Twitter was weak? Microblogging is a good first wedge? Or the creators of the networks were all very online?

clicking @clicker (a @daylight ░░░░░░) | kylemccollom.com

text historically has fit on the blockchain better

need hammock time to think, then i post - follow @1dolinski · challenge your habits · ai isn't new, talk to mine: https://getfirstresponder.com/talk/1dolinski

None of these (serious) networks are blockchains?

yeah this was my first thought as well.  text is a lot cheaper/easier to store than video or images