I'm considering implementing "Login with Facebook" (or Twitter or Google+) but I'd like to see what percentage of our users even have any of those.
(Our users tend to be fairly old and non technical and I suspect most don't have FB accounts.)
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It is possible to know programmatically if you are logged into social web sites.
Check the header (and then the HTML source) of this WSJ microsite to see what other user details besides your current login status on social web sites can be tracked (I found it creepy so I now use a Google Mobilizer Bookmarklet when I visit domains that are not on my whitelist).
Immagine that Facebook will analyze how many users goes in your site through cookies, you'll be happy of it?