The Atlantic  got ahold of some of Netflix's internal stats--nothing with identifiable names, it seems, so this isn't a scandal--and found a wide range of movie ratings per user. Netflix has one of the more advanced recommendation algorithms on the Web, taking your personal ratings for movies or TV shows you've seen and spitting out titles you might also enjoy. Theoretically, the more you rate, the better the recommendations are--and it looks like some Netflix users have taken that a little too literally.
The biggest raters, the "Elite Raters," are those who have rated over 50,000 titles. While those Elite Raters measure only in the hundreds, it's still an incredible time and effort investment on their part. And once you venture down from those, there are lots more who have rated similarly exhausting amounts of movies. About a tenth of a percent of Netflix users, more than 10,000 people, have rated over 20,000 titles, and about a percent, 150,000 users, have rated more than 5,000.
5,000 may seem like nothing, since we're talking about some who have rated 50,000. But only 60% of Netflix users rate movies at all, and the typical user rates only about 200 movies.
Netflix is constantly improving their recommendation algorithm, holding contests to award a million dollars to any development team that can produce a superior system to theirs. That's not arrogance, either--Netflix frequently awards that prize, and the service is better for it.
There's actually an indication that excessive rating is bad for recommendations. Some high raters have pretty much exhausted Netflix's recommendations in their preferred genres, and are stuck with endless suggestions to rent Mystery Science Theatre 3000 movies or whatever. But casually rating movies certainly is addicting, which is in large part why it's so popular.