Possible to estimate the number of users for an Android app?
On AppBrain, you can find the number of installs right at the beginning of the page for the app:



On Google Play, you will have to scroll a little further down, as with the relaunch the summary has moved to a less ideal position:



It's just a raw number of installs done via Google Play, and does not include other sources (such as AndroidPIT, Pdassi, or Amazon). I don't know how is dealt with users who installed an app and later uninstalled it, but that number should at least give you a raw idea.