Almost 18 and STILL not allowed to have a Facebook?

i'll be 18 in three months. i'll be a senior in high school, and i'm still not allowed to have a facebook. i've told my parents time and time again how you can adjust privacy settings and stuff, they know i know about internet safety, and i only have school twice a week so i'd like to talk to people. i just really want a facebook so i can talk to my friends and post my photography. it's really annoying because i also miss out on parties, events, and such. even just little stuff like what people are doing, or when my friends have a new relationship (you didn't know?! me: no, i don't have fb). it's really annoying! and i really DON'T understand why my parents won't let me! :( how can i convince them otherwise?