What about if I click the little arrow next to the address field and click the facebook address that drops down among others?
Do they know where I come from then?
That's fine. It's just hyperlinks from webpages that send referrals. This includes images, scripts, embedded objects (java/flash) and of course text hyperlinks.
Just remember that the cookies for that site may still be cached, so even thought referral information (where you came from) isn't sent, they may still know who you are because of cookies set on your computer during previous visits. That cookie can be read by that site regardless of how you arrived, so they can still know who you are, but not where you came from... unless web beacons are used.
If you want more anonymity, delete that site's cookies BEFORE you visit that site.
As for HTTP_USER_AGENT information, that can't be deleted, but it can be hidden with anti-virus software. USER_AGENT information is just another (albiet less reliable) way to identify a visitor to a website. It basically contains your browser name and version and your OS name and version, and maybe some plugins. This information can be very exact, however, so don't underestimate it. For example, here is somebody's user agent string:
Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.102011-10-16 20: 23 :50
This is very identifiable because very few others will have that same string.
Here is your user agent information. Don't worry, that link is safe.
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