Report them when you see them, there is always a steady trickle and we get them as quickly as they can but occasionally they come in large batches so report them when you spot them so we can nuke them quicker.
Some of them are just bots that go around and signup at every forum they stumble across and post stuff up, others are individuals who get paid simply for spamming ads around. Spammer don't do it because it is efficient, they do it for the one moron in a thousand who follows the link.
They probably aren't bots. You can hire people (even companies) to do this for very little money on freelancing/outsourcing sites. It's often for SEO purposes, not to get people to actually follow the links. Most of them spam sites that don't provide any real SEO benefit (like this one), but when you pay for bottom dollar SEO you get bottom dollar SEO.
Probably not. Every time a UFC fight nears we get hit, everytime a major soccer game approaches we get hit. The ones in the hardware section get nuked pretty quickly so i don't often see many, but the ones in the lower section need others to report them since many of us don't venture down that far.
Wait until a major sporting event comes around, then you will see an increase.