Now that I think about that, neither poll asked takers how much they had donated for existential risk mitigation. (In case you're wondering, the answer in my case would be “zero”.)

What percentage of current posters do you estimate are FAI philanthopists/activists?

I don't want to pull a number out of my butt, but look at this way: our most recent open thread had 287 comments. Our summer fundraising thread had 17. We should probably have a survey.

Can you give a couple specific examples of what distinguishes them from casual users? (donates to SI? works in a relevant field? volunteers for SI? etc)

If they donate a unit of caring to the SIAI (or perhaps the FHI), then I would lump them in the FAI philanthropist/activists category. There are some people who make contributions without donating money, such as Holden Karnofsky. But for the most part, the people who don't give money are freeloaders.

They go to Less Wrong for entertainment, or for practical advice, or for social interaction. They don't add value, and some take value away. They'll direct the conversation to things that are fun to argue instead of things that that might save the world. Or they'll write comments that show off their intellect instead of comments that raise the sanity waterline. I'm glad these people are gaining value from Less Wrong, but if we're trying to save the world, they shouldn't be our priority.

Edit: Changed the last paragraph to make it less mean.