High-quality social network sites grow because contacts are real, and site-mediated communication is welcome. For example, LinkedIn from the beginning treated contact information very carefully, never generating any email except by explicit request of a user. Therefore it felt safe to import contacts into it, since I wasn’t exposing my colleagues to unexpected spam. (LinkedIn has loosened up a bit. Originally one could not even try to connect to someone unless you knew their email address already. They made it easier to connect to people found by search only, and you can pay extra to send messages to strangers; nonetheless, in my experience it’s always user-initiated.)
Low-quality social network sites grow by finding ways to extract contacts from people so the system can spam them, or trick users into acting as individual spam drones. (A worst-case example are those worm-like provocative wall postings that, once clicked, cause your friends to seem to post them also. Just up from that on the low rungs are the game sites that post frequent progress updates to all your friends.)
I’m a joiner and early adopter, but I rarely invite people to use a service they’re not already using. That’s my way of treating my contacts respectfully, and protecting my own reputation as a source of wanted communication, not piles of unsolicited invitations.
Google Plus has recently taken a step toward lower quality by changing their ‘Find People’ feature. Previously it identified/suggested Google Plus users separately (good). Now it identifies and suggests everyone on your contact list and beyond, without identifying whether they are already a Google Plus user. Really they are nudging me toward being an invite machine for them.
As a result, Google Plus will get less high-quality social-network building (among people who respect their contacts and take care with their communication), and more low-quality social-network building (piles of invites from people I barely know). If it goes too far downhill, Google will endanger the willingness of high-quality users to let Google know anything about their contacts or touch their email.