If you don't want to be creepy you could also word the email as a more generic request for feedback like "if you don't use the app why not, if you do what do yo

If you don't want to be creepy you could also word the email as a more generic request for feedback like "if you don't use the app why not, if you do what do you like about it" to make it seem less like you are tracking them although I'm not sure how that would affect the response rate. I think if people know they are being asked their opinion for a reason rather than a blanket request for feedback they will be more likely to respond. As for the 25% I don't really have anything to base that off of, it was just my impression based on how I use sites (I'm one of those that signed up for twitter and never tweeted). If I hear of a site that sounds interesting, but I have no use for their service, I still might create a login and check it out just to take a look.