Lately I've been noticing a new viral strategy popping-up, and I'm not quite sure what to make of it. Here's how it works. You upload your Google contacts to the site, perhaps to find other people you know or as part of some other functionality. (Google works better than other sites because of the way Gmail implicitly adds a contact for each correspondence.)
Then I come in to the site through a referral email. Without entering in anything, the landing page can be tuned to my social connections based on my email's appearance in address books' of my contacts. That is, the service has saved all the previous contact lists so they can make a behind-the-scenes social graph for use in converting me.
Of course when the site is bigger they can use their inherent social graph for most of this logic. But when they're just getting started (or entering new networks), this address book component can really increase their viral coefficient.
This technique isn't definitively bad. You could imagine a big disclosure on the front end (when uploading). On the other side (me coming to the site), it is improving my experience by putting the site into the context of people I know.
But it can get creepy too. I doubt even with seemingly valid disclosures, people realize that their info would be used outside their own account. Also, it can lead to interesting "people you may know" suggestions, sort of the equivalent on twitter of people your friends follow that also follow you, but you otherwise have no shared connections. Those recommendations always leave me wondering, how did they make that connection?
Taking that a step further, the site could continually recommend me to invite people that aren't currently on the site, without me uploading my address book. That is, they know this person is not yet a member of the site and they also know their name and email based on previous uploads. So they could just present the person's name to me, get my authorization, and then kick off the referral email on my behalf without me entering anything.