On a tangentially related note, developers, please remember that not everybody in the universe has a Twitter account, or wants one. If you have some cool product and I must sign in with FB/Twitter, straight to the close tab button I go and I'm not looking back.
And when I decide that it's OK to sign up with twitter/fb there are more things that might turn me off:
1) If the app only gathers some info from my twitter/fb account and still wants me to provide an email/password. Sorry, but I was awaiting something simple like 'click authorize in twitter and receive account'. I didn't expect that much effort and I'm closing the tab then.
2) If the app requests to 'post updates to my timeline' or something like that. I'm using twitter/FB as an authorization provider in this case and have no desire that your app will post updates for me. (Possibly in my absence)
Actually, I'm quite happy to see 'shitty grocery apps' outsourcing authentication, since it means they don't have a database full of plaintext passwords.
When Spotify launched in the US, they disabled signups through their in-house authentication system and required that new accounts be created through the Facebook login. I never quite understood this, since I have a strong aversion to paying actual money for anything associated with my facebook account.. Thankfully they kept the old login system around for existing users (and they've since re-enabled sign-ups through it.)
They were heavily pushing the social streaming aspect, and admitted to as much. Letting other people know who you are listening to is a longstanding tradition, and worth good money to Spotify (who never raked it in to begin with).
I get that, but between the opt-out popups trying to trick you into broadcasting stuff on facebook and disabling signups for the old login system it got to be a bit much.
While I agree it's dumb to only provide a FB/Twitter login, you should also keep in mind that I am much more comfortable with logging in on Twitter, mostly because most things I post or do on Twitter are implicitly meant for public consumption (ignoring things like DMs, which feel like afterthoughts).
Facebook gives the option to the users not to give some permissions. i.e. in theory an app is not allowed to require some extended permissions such as post on your timeline etc. Of course in practice I've seen apps that won't let you continue unless you give them all permissions they asked for - which is against the rules Facebook set in place.