I'm not sure, see I wouldn't say even dating, as I've been on my first "date" with one of my girlfriends about 3 months after starting becoming a couple.
I would've said slept with, but then I had a few one night stands.
Facebook still makes it obvious when a couple is a couple because its when you both agree to show you are in 'something' with the other. If memory serves it asks you to confirm if you want to show your connection (haven't had a situation when the rel' status warranted changing since the big breakup of naughty-seven)