Clicking the Like button falls definitely into the "actively uses Facebook" slot, but commenting on 3rd party sites is indeed a gray area. But I'd be surprised if there is a lot of people who use Facebook only that way. Anyone has any numbers that could shine light on this?
Does it really? Does everyone that sees a Like button actually understand what it does? I suspect not, and that a bunch of people click it just because they like the site and thing it looks fun to press. The same logic would say that anyone who hits a +1 button next to a search result is a Google+ user, which is equally silly.
I agree that numbers would be good here: how many Likes (or +1's) are hit by users without other recent activity?
