As of October 1st, Facebook announced that any iFrame page tabs not hosted on a secure server under HTTPS will not be displayed to users browsing under HTTPS.
Does your Facebook page or your fave sports team have a custom tab displaying this?
It is easily fixed by ensuring your iFrame code works under https, previously this was optional not Facebook wants to provide a more secure platform requires https.
Have you found a broken Facebook page?
Let us know in the comments which Facebook pages have non-secure tabs.