Well, I would (should) change the database class and router to be DI, but as for the other (few) singletons, they aren't used that much and are only sprinkled in places. I prefer the cleaner look with lazy-loading, just my opinion...
I do yes, but there is so much to add at the minute that it would probably not be worth it this early, I would be forever hounding you to update! And thanks, you give me hope saying that!
I had some servers built to my spec and testing showed that everything does a pretty damn good job, several thousand concurrent visitors, all sending requests every couple of seconds. The code was the bottle neck though, had to move some stuff to other places, even still the hardware has plenty to give.
I think (I hope) the current capacity will be enough until there's enough paying customers to warrant/pay for upgrades.
Thanks, functionality wise I plan for it to be a hybrid between high level traffic and low level events like those you mentioned. The key features at this stage are, with the exception of campaigns, everything can be manipulated retrospectively, so there's no problem if you forget to do something. Where it's economically feasible, there can be raw data manipulation on the database for you, an actual human to talk to, custom reports, basically trying to bring and enterprise level service to the smaller guys, we'll see how that pans out...