I haven't seen many Flash games on facebook or other applications really utilize it yet (or appear to utilize it).
Are there some good professional examples of its use?
Is pixel bender just too slow for practical use on today's web hardware? I read that Flash will fallback to interpreted mode on older Macs (PowerPC), for example, but is that a deal killer?
I have used them for games and websites of motion picures. For example the official site of Harry Potter 7, Part I. And also for some promotional games for Macgruber, and for an AIR app that promoted the movie Terminator Salvation.
They work great, and in many cases can be significantly faster than doing the same effects in another way. But have a very specific use case. I think one of the main reasons you see so little of them is the programming of new shaders requires skills most AS developer do not have. Its hard, and its entirely procedural.
So I cannot account for their being widely used, but theya re definitely a good feature that is production ready.