I'd agree with all of the above ..except this part
They spam at odd hours of the night because that's the working hours in the country of origin, not for any strategic reason.
in relation to here at WebmasterWorld..there is one forum ( not yours:) where I see the spammers hit it, and only it, between 4 am and 7 to 7.30 am Paris time..
the particular mods are both ( I think ? ) in the USA asleep at that time ...
The spam relates precisely to that forum..they don't spam any other forum here..
However at that time of "night/day".. I'm around quite often ..as are Bill and Incredibill, and Tedster..so it doesn't last long ..but they still hit that forum almost daily,and only that one, always in the same time window, always very relevant spam to that forum..
always with "sign up links"..
If I'm working on something on another machine, I leave the active threads tab open and an aubible "pinger" on this box, when a new post is made here, soft chime goes off.. ( also does it when the "ads" at top right change )..when it pings that time in the morning..50% of the time..it's a spam post of a "sign up" type in that particular forum....agreed.. they are usually foreign ( or have slightly "disjointed English" )..but usually ( 99% of the time, as is most spam ) spamming for USA or far less often UK based businesses..