How can Facebook know I have more than 1 account?

[C]

I make separate facebook accounts for my own personal reasons. How can Facebook know I have more than 1 account? I have like 3 and one of them is requiring me to use a cell phone number.. How can I prevent Facebook from figuring this out?

[Devin Halbert]

This is generally caused by similar account name log-ons from the same Internet Protocol address or same Internet Connection(also cell phones). I once had around 10 accounts just for FarmVille boosting,but that was 2-3 years ago and Facebook has increased security to prevent multiple account creations.