Nice idea, however, visits by search engines often amount for most of that number. For instance at the moment xmarks shows

"132 (38 members & 11 guests & 83 bots)"

as the "currently active users". The "bots" are search engine spiders. You can see more detail of which search engines cause this at the end of the "currently active users"

"Yahoo! Slurp (81), MSNBot (1), Google (1)"

That means that 83 of those 130 "users" are actually search engines. For comparison the TOTAL number of registered users visiting the xmarks today is a (still impressive) 279. The majority of the gap to the 857 peak number is likely to be accounted for by search engine traffic.

So, unless you have a way to influence the timing of spidering of search engines, then it will be very hard to coordinate a record attempt. Sorry!

KP