Good question!
Number of Currently Active Users
Since site visitors can read the forums without registering it's common for there to be many "guest" users in the forums. That includes anyone with a bone marrow failure disease who wants to read the forums but doesn't (yet) want to register and make their own posts. The "guest" count also includes search engines like Google and Bing that scan Marrowforums to update their indexes. There are typically a couple of these search engines visiting Marrowforums at any given time.
Most users ever online
Sometimes there's a surge in traffic just by random chance but other times we know what caused a wave of new visitors and new forum members.
The record-setting surge on January 9, 2011 resulted from Marrowforums being mentioned in the
January 2011 issue of The MDS Connection (a publication of the Aplastic Anemia & MDS International Foundation).
Marrowforums was also mentioned in the
March 2010 issue of the eInsider newsletter, the
June 2011 issue of AA&MDSIF INSIDER, and the
November 2011 issue of The MDS Connection.
Finally, there's a link to Marrowforums in the footer of each issue of eInsider, which brings people to Marrowforums on a regular basis.