From this page, it looks like every drop.io user already has a magic key that they could copy and paste into a migration tool.

  http://dev.drop.io/

Assuming that their userbase is large enough for some percentage migrating to benefit you materially - consider building a migration wizard now.

This sounds like a talent acquisition - I doubt drop.io would be hostile to your poaching their about-to-be-ex customers. They may even be willing to suggest your tool to their users.