Second Life could put ads on social network sites like facebook, MySpace, Bebo, etc.
Join SL from such an ad and you’d have the opportunity of signing up using your display name at that site as your first name in Second Life, with the last name being facebook, or MySpace, or Bebo, etc.
You’d be able to invite your friends from that site to join Second Life.
There’s be official inworld groups for each site that had such an ad. You’d be offered the chance to join the group or perhaps just be automatically entered in the group.
There could be an orientation island for the folks that enter from such a site.
The snapshot system would be modified to enable posting to the site one joined from.
Thus one could join Second Life and find friends that they know from other sites.
Existing Second Life members that wanted to join one of the social network groups could do so after a validation process linking their SL account to an account at the particular social network.