I'm seeing this same issue. Keep in mind this is not a supported portion of Tableau, but a third party sample. So unfortunately, these things can happen. It's possible that there is a bug in the connector, or maybe even Facebook made a small change to their API.
I'll ping Alex to see if he would be gracious enough to devote some time to fixing this If you are interested in attempting a fix yourself, I would recommend using the developer simulator instead of Tableau, as it will be easier to see logs and see where the connector is getting stuck.
Yeah I've tested with developer simulator (this uses the 1.1.0 JS so you have to use the legacy simluator), And it assigns the fields, but never seems to actually pull data. It just assigns the data columns, and then sits. I spent a few days with it and finally gave up and started working on a Google Sheets solution instead. If We could find a solution I'm sure I'm not the only one having trouble. And so many examples here seem to use that connector.
So for anyone else who runs across this, my final workaround is to use supermetrics through Google Sheets to pull the data I want.
Now that Tableau Ten supports Google sheets native is even a little easier.