[tomstokes]

    Flock uses new battery-friendly location technology and
    sophisticated algorithms to magically know which of your
    Facebook friends you are with when photos are taken.

My bet is that they're using the background significant location change service. Each time iOS wakes the app with a significant location change notification, Flock combs through your photos and sends their GPS coordinates from the EXIF data to the Flock servers.

What I'm really concerned about, though, is that to pull this off the app must send your location to the Flock servers for correlation with other users' photo locations. This means Bump now has a reasonably accurate location log for everyone using the Flock app.

I wrote a test app for the iOS significant location change service that logged my location for a week, and it was surprisingly accurate. Enough to clearly tell when I was at work, at home, driving down the freeway, etc.: http://www.tomjstokes.com/130/iphone-ios-4-2-background-loca...