How can I convince my parents to let me have a Facebook?

[ǝıʇɐʞ]

A few months ago, I asked my mom about it, and she said if my step dad (who is EXTREMELY strict) says yes, then I can have one. The whole thing ended in me being grounded and balling my eyes out because my (step)dad insulted my grandfather and biological father. (it was horrible) I don't understand what the problem is. I'm not going to be on it all the time. I'm not going to add random people. I want to add my group of friends, and close family members. That's it! How can I convince them to let me have one?
Please don't say I'm selfish or spoiled or anything like that. I don't NEED a Facebook, but I would really want one.