Not sure how this is calculated, but you're estimating Instragram at 4B+ and Google (maps, mail, chrome, voice, ...) at 800m? Seems like something's wrong.
In aggregate, google's apps have more installs -- but many are installs by the same user, that use gmail, google maps, youtube, etc. Also, the value is about the apps, not the companies / engineering teams / brand recognition behind them.
Instagram has actually been historically dropping in our valuation as the growth curve dips down, especially on iOS.
