Does anyone have thoughts on using flash for facebook pages?
In my humble opinion using Flash in custom tabs on Facebook pages is not on optimal solution.
First of all it takes already quite some time for the content of an iframe in a page to load, so adding a plugin to that will only extend the time it takes for the page to load completely.
There is one domain thought where Flash is very popular on Facebook: social games. A lot of them are written in Action Script, and Flash makes it easy to develop these games pretty fast.