I've never received that prompt. If you read further down in that misleading article you linked to, you would see that they don't show the prompt to all their users:

> 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.