[AlexMuir]

I've seen quite a bit of this - and even put considered developing a similar service (justseen.at - domain for sale).

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.

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.

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).

In the end I decided it wasn't something I wanted to be involved with from a reputational standpoint.