SO just hit 250K questions... congratulations! Is there a # of questions vs. time graph? Just curious. (is it O(t) or O(t log t) or O(t3/2)? ;)
Here's one I just made, plotting posts per day vs. age of site. Actually looks more like O(log(n)) to me, or somewhere between O(n) and O(log(n)), but I'm no statistician. The big spike is the public launch. The dips correspond to American holidays: Thanksgiving around 120, Christmas around 150, Easter around 250, Memorial Day around 300. See a higher-res version here.
I started with this query:
SELECT DATEDIFF(FROM_UNIXTIME(creationdate), '2008-07-31') AS age , COUNT(*) as num FROM posts GROUP BY age ORDER BY age
Then I separated weekends from weekdays, since they were clearly two separate trends.
Update: As requested in the comments, here is a chart showing total number of posts vs. age of site. I'll give you that this looks like it could be O(n2), with a very small constant factor (and an anomalous bump around the public launch). Unfortunately I don't remember enough about statistics to tell you which function is a better approximation. I know you need to use r-squared and rho and standard deviations, but I don't remember the significance of any of those things. One of them is supposed to be less than 1.0 I think. Anyway. A higher res version is available here.
Okay, just for fun, here's one plotting posts per day, grouped by day of week. I'm surprised Monday is consistently lower than other days of the week. Higher res here.