In the spirit of https://xkcd.com/1053/, I'll assume you haven't heard about the privacy violations, misleading or hard to find control panels, and constant changes to visibility settings leading uninformed users to unwittingly post sensitive information with public visibility.
Facebook (the web app) has its uses and reasons to like it, but there are also lots of reasons not to like it.
I truly think a redesign is the only thing keeping from Twitter today to Twitter 10x. It is too cluttered and they are innovating their design too slowly. Remember how fast Facebook released changes (albeit facing massive protest)? People got over it and they are still growing. Twitter needs more contextual filters!
It is the top comment and having a username+password option is still reassuring even if you choose to go with facebook or another third party anyway. Many will only use facebook to login if there are alternatives.
