Reader Stephen F. noted in an email to me that when you go to Facebook on an iPad this line of code appears: <meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0; user-scalable=0;" />
It's the user-scalable=0 part that disables zooming on the iPad. Stephen also pointed out that you can test this by changing the user agent on your web browser to iPad and seeing how the code only appears for the iPad. To change the user agent in the OS X version of Safari to emulate Safari for iPad go to Safari>Preferences>Advanced>Show Develop menu in menu bar, then select Develop>User Agent>Mobile Safari 3.2.2 – iPad.
In short, Facebook has either deliberately or accidentally disabled zooming in mobile Safari on the iPad. If it's accidental, it's odd that they haven't fixed it in almost 72 hours. If deliberate, the question is why? Was dinner that bad?
UPDATE: Zooming works again just fine again. I contacted Facebook PR about the issue, but they didn't reply.