Instagram opened up its API recently to Hipstamatic [1] (which I think predated Instagram, not sure though). Interesting to see Instagram realize that their value is in their userbase and not in their specific filters. I don't know if they have plans to become a platform or not, but with millions of users (and with this Android release, millions more coming), they've definitely got that as an option.

[1] - http://www.fastcompany.com/1824797/exclusive-hipstamatic-ins...