I don't have a lot of data on number of sites using the FAQ plugin or number of questions being maintained but it has been downloaded a few hundred times.
The forum plugin is widely used and I know of sites with 10-20,000 posts and 100's of users.
Both are excellent components for Company Intranet or business applications as well. I hope that if you are using them for business you will stop by my site and contribute as that will help me support my projects.
Cheers,
Blaine
Remember Opensource does not mean free