[rplnt]

Third party sign in might not be ideal, but having to register for every crappy service and website is much much worse.

And flickr or tumblr have quite a bit more users than you've guessed.

[blueskin_]

That's why people have KeePass, or even LastPass if they are overly trusting of third parties.

Having a different password for every service is more secure than one for everything.

[rplnt]

I'm not talking (just) about security. It's about convenience. It's annoying to go through million variations of registrations instead of approving the site with one click.

[blueskin_]

1. Enter username, and email address if not used as a username.

2. Enter password

3. Click link sent to email address.

Seems more convenient that having your privacy steadily eroded and introducing a SPOF.