One glaring omission from Google+ has been the lack of third-party app support, preventing you from sharing activity to the service within apps. Today Google has released sign-in for Google+ so developers can offer greater options.
Google+ Sign-In must be integrated by developers, so currently you'll only find it in a handful of apps including OpenTable, Fitbit, Flixter and Shazam. That said, other integrations are underway and hopefully we should see additional options in the near future.
Google took a while getting this feature ready because it didn't want to allow third-party apps to cause "social spam," or the constant stream of pointless information you often find in the news feeds of your Facebook friends. Additionally, Google gives you a lot of control over how apps can interact with your account and you can adjust the settings to your liking. Simply visit this page to keep an eye on any apps active on your account to keep the working the way you want.