[DamnInteresting]

If you're not using a plugin such as Facebook Disconnect, pages that have a "Like this on Facebook" embed or similar can accidentally or deliberately reassociate you with your prior identity. Consider this scenario:

1. You log in to your account and get redirected to http://example.com/?user=3834

2) That page has an embedded Like button.

3) When your browser requests the button from Facebook, the referrer is "http://example.com/?user=3834" which is a URL that you visited a lot when your old Facebook login was active, and it was never visited by any Facebook users apart from you.

There are other similar ways they could link you to an old identity if they wanted to, some not necessarily blockable by these plugins, but the above would be simplest.