The proc does stack, but only if there are multiple hunters with 8/8 in the raid. (In other words I can't stack it by myself. I'd just refresh it.)
It's around a 10% or so proc, I'd say, on any direct damage ranged ability (so serpent sting doesn't count). This includes stuff like arcane shot and scatter.
How can you help?
I can shoot things and then make my pet move toward them.
PPM's are based entirely off the equipped weapon attack speed. Anything you can do to speed up your attacks, or add more via instants just results in more procs that the 'baseline'
I've been getting vastly different proc rates both here and from questioning my hunter friends.
I'm not looking for PPM's, I'm looking for #procs/#hits. More to the point, I need #hits to be a very large sample size. I'm sure that there are lots of DS8 players out there with procwatch or some similar addon. I'd go out and shoot 6k+ arrows myself, but I don't have a full set just yet.
If people with proc data on Expose Weakness would be so kind as to post or PM me their #hits and #procs, it's trivial to pool the data. Hopefully, the sample size will end up being large enough to get the results within the 95% Confidence Interval.