Facebook and Microsoft have acknowledged a problem when using IE11.
Until they get it worked out the recommended workaround is to enable Compatibility view for Facebook in IE11.
Problem is, you have THREE factors involved here.
1. Microsoft, who now embeds their OWN modified Flash Player in IE.
2. Facebook, who has to keep up with three OS's (Windows, Mac and Linux), and more than a dozen different browsers.
3. Adobe, who makes the Flash Player for three OS's and more than a dozen different browsers.
Guess which two Adobe CAN'T control?
As I posted, BOTH Microsoft and Facebook have acknowledged the problem with IE11 and Facebook. That's where the problem lies. This wasn't a major issue before IE11 was released.