If you add your social profiles to your google profile, Google will add all your contacts in those social networks to your "social circle". Also your gmail contacts, or people you follow on google reader and so on.
Gmail has started suggesting friends to 'consider including' on emails. Personally, this has resulted in both me and my ex getting some very uncomfortable suggestions recently for whom to consider including in correspondence. It hasn't suggested we email each other, which would be forgivable, but a third party whom I had never directly emailed. And it highlighted for me what I consider to be a major problem with Google's social initiatives, which is that they've collected and employed this 'social' data in a completely antisocial way--behind your back, when you're using unrelated services. And it might be clever, but it can get very creepy.
And this is one of the pitfalls of testing things internally at a company like Google.
There are a lot of important use cases in social networks around "me and my recent ex", but within a corporation you'll be unlikely to run into them. (Similar issues caused a lot of complaints for Google buzz.)
I don't think the suggested recipients feature pulls from other users' contacts. You, your ex, and the third party have probably been recipients of the same message in the past.
Yeah, I don't know how you define friends. They already show "Foo shared this on twitter" for some results (with a link to the twitter profile), which I've found quite useful.
From the article: "Right now, the only way those within your social network on Google will see what you’ve +1′d is if you have effectively created a fresh network just for this, deliberately chosen to expose your +1 activity, and if you’re friends know exactly where to look to see this."
