Many services push users to log in/register using their Facebook credentials so they can get additional demographic information about them. Only if you absolutly don't want to sign in using 3rd party credentials will these sites begrudgingly allow you to register using the traditional pick-a-user-name-and-password form.
StackOverflow has a particularly cunning way to get you to register using your Facebook credentials.
StackOverflow's signup page starts out like many others -- they strongly encourage you to sign in using Facebook, but also put a link to their traditional sign up page
Generall a private person, I usually decline to sign in using my Facebook account, so I navigate myself to their traditional signup page. It looks like a standard signup form at first, but then I quickly realized their password requirements are just plain ridiculous. Your password on StackOverflow needs to:
- Have at least 1 uppercase letter
- Have at least 1 number
- Have at least 8 unique characters
Really?! Have at least 8 unique characters?! Even my bank doesn't require this much complexity in a password. And we're talking about StackOverflow here -- a site that doesn't really store any valuable personal information like credit cards.
Discouraged by having to invent a completely new password just for this site, I decided to
hit the "back" button on my browser and just sign in using my Facebook credentials.
This seems like a brilliant tactic to get more users to sign in using 3rd party credentials. I wonder what percentage of SO users sign in using Facebook, and how this compares with others.
EDIT: I was informed by someone at Stack Overflow that this in fact was not their intention at all. Futhermore, they don't use any of their user's Facebook profile data. I now realize it was a bit of a stretch to say that Stack Overflow engineered their UX in this manner.
Regardless, the strong password requirement DID convince me to sign in using my Facebook credentials, and I'm sure it will convince others as well. In conclusion, I think a very strong password could actually funnel users into signing in with 3rd party credentials.
I should also say that I'm not complaining about SO's strong password requirement.
EDIT 2: I also didn't realize until now that Stack Exchange is an identty provider, in which case the strong password requirement completely makes sense. You shoud probabaly just disregard anything I said in this post, as I clearly didn't know what I'm talking about :)