User blog comment:DutchRits/Character Card Comparison, Part 7/@comment-13278317-20130815212137/@comment-15167314-20130816065411

Android users sometimes get offers to try other Mobage games in exchange for Mobacoins. It takes a while to accumulate enough coins to actually get something, but you can save up enough for at least one trip through the Space Bridge.

There's an app called FreeMyApps that lets you do the same thing, indirectly; you rack up in-app points that can be applied towards Google Play gift cards. It takes a lot of points, so this way takes a really long time. They only give away gift cards worth $10 or more, so you can get a lot of coins this way.

I haven't tried Tapjoy, but I ought to look into it.