This graph attempts to illustrate the impact the One True Thread of Time has had on the overall activity of the xkcd fora. The graph displays, day by day over the course of Time, what percentage of fora posts were made to the OTT.
The first few days, of course, are the busiest, followed (as I've noted elsewhere) by a big drop after the One True Comic did not end on April 1st (the end of Week 1). There is a fairish slump during the fourth week, and another big surge around the Fading and the beginning of the exploration period. But since the sixth week we've been bobbling along fairly steadily, accounting for between 30% and 45% of all xkcd fora activity. I don't see any particular pattern corresponding either to weekends or to Othercomics, except that Mondays perhaps slump a little more often than other days.
Post numbers assigned by the xkcd fora (the "p=" number appearing in the URL when you click on a post title or that little 
icon next to the username) appear to be sequential, and unique across the entire xkcd fora system. You can find a post by post number alone, without a thread reference: The post by macraw83 that inspired this graph, post #3402599, is at url http://fora.xkcd.com/viewtopic.php?p=3402599#p3402599. Post #3402600 immediately follows it, but post #3402601 is in the "Create Your Own Nation" thread in "Forum Games", and #3402602 is in the "Multiverse Mafia Day One: Joined Realities" thread of the "Mafia" forum. The sequence of post numbers within the OTT indicates that posts are numbered when they are submitted by the user, not before; posts which are created but never submitted are not numbered.
So I used a simple technique. Subtract the first post number of day n from the first post number of day n + 1, and that's the total number P of posts for day n across all threads on all fora. Directly count the number of OTT posts T on day n. Then T / P is the fraction of day n's fora activity that was generated by the OTT. (This calculation ignores the effect of deleted posts, which, since I'm far too lazy to measure it, should be relatively slight.)