It fluctuates. By far the most popular time period was the 3.55 jailbreak. I don't think we'll ever see that again unless this happens to the PS4. Normally though it's about the same each year. During this time of the year things slow down due to school starting up again, and then it picks up around Christmas and again during the spring.
The thing about NGU is that the most active parts are very specific sections. For example you rarely see more than like 5 people in this section at any given time, while GTA sections can have up to 100+ registered members even later at night. Same thing with the CoD sections. However I hope once the awards get finished we can add awards for posting a lot in the less active sections which might help spread activity around.
That's definitely a cool way to implement the award system. I better be the first to receive an award related to Counter-Strike. :p