I've seen quite a bit of this - and even put considered developing a similar service (justseen.at - domain for sale).<p>The first problem is one of being perceived as invading privacy. This creates all sorts of follow-on problems - Apple won't allow you in the Appstore, Google Maps will likely withdraw your permission to use that (as I think happened to Gawker). There are ways around these, but it makes it a murky business and complicates bringing in revenue.<p>In the UK (where celebrity magazines are huge), Papture have been doing all they can on this. There's not much hope of an exit - mainstream celebrity magazines would probably worry about stepping over the line in acquiring something so intrusive.<p>Potential revenue sources that I saw were paid-for SMS alerts whenever a particular celebrity is spotted (imagine the number of teenies following Justin Bieber). And perhaps paid for accounts allowing real-time access, while everything else is delayed by 2 hours (for safety reasons).<p>In the end I decided it wasn't something I wanted to be involved with from a reputational standpoint.