F's
FAAK - Falling Asleep At Keyboard
FAQ - Frequently Asked Questions
FC - Fingers Crossed
FD! - Funk Dat!
FOAF - Friend Of A Friend
FOFL - Falling On Floor Laughing
FMN - Forget Me Not
FTBOMH - From The Bottom Of My Heart
FTP - File Transfer Protocol
FUBAR - Fouled Up Beyond Repair
FWIW - For What It's Worth
FYA - For Your Answer
FYEO or 4YEO - For Your Eyes Only
FYI - For Your Information
Okay, this one has alot of different meanings I will list the ones I have seen used in chats
FU or FY - F_ck You, Forget You, For You
choose the one that best suits the conversation. Some might not know and take it offensive so be careful with your choice of letters.