When an app is installed, it can be added to the system-wide share menu so that any app capable of sharing content can use it as a destination. Even though Instagram doesn't have G+ sharing built in, I can just tap the share button on any photo and send the photo to my stream.
Reshared post from +JR Raphael
A lot of people take this feature for granted, but when you spend any time using other (ahem) more restrictive mobile platforms, you realize just how valuable it is.
Android’s (quiet) killer feature
It’s no secret that Android has lots of good stuff going for it, but one of the platform’s most useful and distinguishing features is one you rarely hear discussed.
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