Suppose I have a tag on both porno.com and kittens.com. I can fingerprint you across both sites, even if porno.com is incognito. Then you can tweet on kittens.com and I can now tie your kittens.com usage to your twitter usage.

I only need you to engage in one of many non-obviously trackable social interactions ("YES send me a cute kitten every day to my email!") on kittens.com to identify you.