Ask HN: Name a good location-based game platform.

I tried asking this yesterday, and it got buried pretty quickly... so here's a second try with an update.

I design location-specific scavenger hunts and themed games, and I was recently asked to design a game for a mall. They also asked if I could put it on a mobile device for people to download, and I said, "Of course!"

Oops.

SCVNGR is shuttered, and I only recently found a few additional tools. I'm not sure how designer friendly they are, and it would be nice to have others' opinions on them:

playground-thursday.com/

joyity.com/

placechallenge.com/

I don't know of any other platforms, either, but before I commit to one, it would be nice to hear about them from a designers perspective.

Thoughts? Suggestions? Critiques?

Much appreciated, -s