Shorthand for Face to Face, F2F or face time (not to be confused with Apple FaceTime) is used in chat to let someone know you would like to meet him or her in the real world and not just over chat or the Internet. F2F can also be used when telling someone else that you've finally met someone in real-life. Below are some examples of how this could be used.
I enjoyed talking to you, would you like to meet me F2F?
I finally met Jessica F2F, and she is beautiful.
Tip: When meeting someone from Internet it is a good idea to meet in a public location, take friends with you, and let others know, especially your parents if you are under 18.