Social bookmarking which seems to justify small group interactions have now led to more exponential growth as your friends' friends and friends' friends can all be reached out. As a result if you have an account in any social media site, you can, potentially, get an invitation from a total stranger.
Now step a few years back when telephone marketers would make call offering products and services and were seen as annoying. Social bookmarking, in contrast is seen as "cool" and "coolness" is measured by your list of friends or followers on twitter. Remember, a few months back, CNN was trying to compete with Christopher Ashton Kutcher aka Ashton Kutcher for number of followers on Twitter.
Secrets of Social Bookmarking