I do not trust comscore's numbers at all. They rely on opt-in sampling of user data. There is obvious selection bias there. Large companies will never allow such tracking software, so anybody browsing from work will never be counted. There are many other large demographics that they just plain miss. They have a history of goofing badly, for example being directionally wrong about Google's ad revenue [1].
To me it seems obvious that Reddit's core userbase would not be willing to install the spyware that comScore piggybacks onto to acquire its data. It doesn't surprise me in the least that Reddit is consequently under-represented in comScore's numbers.
The numbers that they show (the same place the one-billion pageviews stat comes from) indicate that they're now closer to 14 million monthly uniques:
http://www.reddit.com/tb/fdyyf