I'd be surprised if the number was as low as 80%.
> we're displaying this prompt when a user who has not enabled secure browsing (through the account settings option) manually changes their browser's address bar to https://, which does not fully protect their Facebook traffic.
I just had to go hunting through privacy and security settings for 5 minutes to figure out how to enable HTTPS.
So no, I would guess fewer than 0.1% of Facebook users have this enabled.
I thought the same thing as parent. It seems the poster is suggesting that tons of people are using https, and I can almost certainly say that just about everyone I know does not use that feature.
I believe the poster of that facebook post (Limited Run) said they would post their analytics details or something like that. I'd kind of like to see them, myself.