You're worried about someone adding your boyfriend on Facebook? Seriously? Yes, you're overreacting. People add pretty much anyone they can think of on Facebook. I have a small Facebook friends list, and even so it includes several people I barely spoke to at school, two exes, and my university director of studies. Some people I have furtively taken off my reading filter, as I don't hugely like them but don't want to kick up a fuss. And I'm 33 and relatively conservative about social networking.
When you say that you trust him but not her, are you suggesting that she would stalk him? Or are you saying that he could never be unfaithful, but that she could talk him into it through the medium of social networking? Come on, sweetheart. I know it can seem odd when exes pop up like that, but short of the occasional nutters who do actually stalk people, they're really not going to change anything about your relationship. And of course, if he were disposed to be unfaithful, then something like this wouldn't make any difference, and you trying to limit his online activities could make things worse.