Well, I just started doing reviews in August, so I've got a few more products to review that I currently own that EF sells. I guess I don't HAVE to review them since they weren't a part of the review program, but I want to.
That probably determines how many people write as well. If they only write the reviews that they are sent a product specifically to review, then it won't be as often. Then for those who write reviews for everything they have that EF carries, it'll be a lot more.
Also, if they are employed, or attend classes full-time.. I guess it depends on alot of things really.
Me, I am currently unemployed and I am only taking one class, so I have extra time on my hands to do more reviews. I also tend to procrastinate my other things I need to do and instead come online here. Right now I have my sociology book sitting on my lap but instead I am replying to this thread lol! Sometimes I choose to write a review over doing something else that really should be getting done (I know, its terrible!).