If imitation is the sincerest form of flattery, both Facebook
should be blushing from cheek to cheek. After the introduction go Google+
, the two social networking services have been copying features from each other, inadvertently helping the other to innovate and improve upon their own social playground. It's still happening, too. Witness, for example, a new feature on Google+ in which posts that are +1'd by people in your circles will appear on your feed.
Facebook already does this by highlighting posts your friends like. On Google+, the person or people who +1'd a particular post that shows up on your feed will be displayed above the highlighted post. In doing so, you're exposed to content that you might have otherwise missed, namely posts that appear on feeds of friends of your friends. You might also end up adding new people to your circles, as well.
This will work in reverse order, too. In other words, if you +1 a post, it will be shared on your friends' feeds, unless you choose to keep your +1's private. In addition, any posts that you've restricted sharing to just your circles will not show up outside of those circles, even if they're +1'd.