Facebook has acquired Instagram for nearly a billion dollars in cash and in stock. Before Instagram reached a Android app it had reached nearly 30 million users. Mark Zuckerberg, Facebooks founder stated "This is an important milestone for Facebook because its the first time we've ever acquired a product and company with so many users." He had also said that he is not planning on doing many more of these. The whole reason Facebok bought Instagram was because they didn't want to loose their users for a picture app. Due to the fact Facebook is all about pictures they had a big competition when Instagram came.
http://gigaom.com/2012/04/09/here-is-why-did-facebook-bought-instagram/