Google has an awkward way of dealing with contacts in its communication apps. In Gmail, your contact list includes all the people you've ever replied, but you can also add other contacts manually.
"Email addresses are automatically added to your Contacts list each time you use the Reply, Reply to all, or Forward functions to send messages to addresses not previously stored in your Contacts list," according to the help center. While this should save you some time and effort, your contact list will include a lot of people you wouldn't have normally added. For example, if somebody sends me a tip for this blog and I reply to thank him, my contact list includes that person. Gmail doesn't have an option to turn off this feature, so all you can do is to either ignore your contact list or create a group that contains only your real contacts.
By default, if your conversation with someone includes more than 2-3 replies, that person is automatically added to your list of Google Talk friends. To chat with someone you normally need to ask for permission, but this feature bypasses the annoying question because Google assumes you really know that person. Fortunately, you can disable it in Gmail's settings and Google Talk, but not many people will do this. "If there are other Gmail users whom you frequently email, you'll be able to chat and see each other online without having to send an invitation. Gmail automatically determines which contacts you'll be able to talk to without having to invite each other." (Gmail help center)
So the rules are simple:
if you reply to someone's email, that person is added to your Gmail contact list.
(Error #1: you may not know that person)
Rule #2 (opt-out):
if you reply to someone's messages more than 2-3 times, that person is added to the list of Google Talk friends.
(Error #2: see error #1. Also that person may not be your friend.)
So it's quite likely that your Gmail contact list and Google Talk friends list include people you don't know. Now that we have these two lists (obviously, Google Talk friends are also Gmail contacts), you may wonder where you could use them.
In Google Shared Stuff, a rather obscure social bookmarking site, all the web pages you bookmark are public, but there's also a page with "stuff from people you know". That page shows the most recent bookmarks from your Gmail contacts, but many of these contacts are people I don't know.
Google Reader added a feature that shows shared items from your Google Talk friends. Here's how it was introduced:
One of my favorite uses for Reader is to share interesting stuff with my friends. I click "Share" whenever I find an interesting item, be it hilarious or serious. This way, all my friends can subscribe to my shared items (and I to theirs), and we can easily see if a friend has found something interesting. This can be inconvenient, as I have to distribute my shared items link to my friends and vice-versa. So, we've linked up Reader with Google Talk (also known as chat in Gmail) to make your shared items visible to your friends from Google Talk.
Except that, according to the rule #2, my Google Talk friends aren't necessarily my friends.
Google will probably continue to use your contact list for other services, so at some point your Gmail contacts or Google Talk friends might see your public documents, photo albums, notebooks, personalized maps, blog posts. All of these public actions dynamically generate a news feed (the way you know it from Facebook) and your contacts should be entitled to find things about you. The main problem is that your contact list has been generated automatically and has little to do with you. Those people aren't necessarily your friends, your family, your co-workers, they're just some people you happened to email at some point.
Before using theses random lists of people to broadcast information about you, Google should clearly define their purpose and let you manage them. The problem with creating a social layer over Google's web apps is that Google is not a social network and your contacts are not your friends and not even people you know.