Branching off of the site plot vs sandpit discussion because I think Achera brought up a decent point.
I've found the same thing, it's why I have a zillion characters on these kinds of sites because otherwise I get bored. I had thought that maybe you want to limit how many people are involved in any one subplot (with the idea that the subplot forwards the main plot in some way) but to make sure that everyone gets an equal chance of being involved, you have several subplots going at once. I can imagine that running into the same issues however, not to mention it sounds like a lot of admin handholding to me.
So how do you manage your site plot? Do you have any tactics to ensure the site plot moves forward at a steady pace or do you just accept that it'll move forward very slowly? (Maybe that's your preference!) How about getting people involved? Do you struggle with it? Do you have any tactics to make sure people get involved or can get involved?