Note: Marco Polo has been sold to Libera Inc. and I will no longer be developing the Marco Polo app. I'm very excited to see what they will do with it! (2 May 2013.)
Original post (16 June 2011) follows:
So how do I use this?
Start by adding your friends' Facebook, Twitter, Foursquare or Flickr account. Then hit the 'refresh' menu button to download their latest checkins. You can then view the checkins on a compass or Google map, along with your current location.
How come I don't see anything?
Maybe your friend hasn't made any geo-tagged posts in a while. Marco Polo only downloads posts with location information: Facebook checkins and Twitter posts that show a little pin in the upper-right corner. You can also configure the maximum age to look for posts in the settings.
Hey! How come you spam my friends when I add them?
This is another attempt to allay privacy concerns, so your friend knows you're following them, and they can see what MarcoPolo actually allows you to do. And a little bit to self-promote the app :) The paid version might include an option to not send these notifications. Note that if you add multiple accounts for the same user, only one notification will get sent.
Isn't this sort of creepy?
Not really. You can only add people to Marco Polo if you're already friends with them. So you can't follow people who aren't already sharing their location with you through their respective social networking sites. Hopefully your friends aren't sharing their location information willy-nilly if they want some privacy.
That's it? It's sort of lame.
Maybe. It's just a toy right now. I have plans for future enhancements that will make it a lot more fun. Stay tuned!