Facebook certainly benefits from network effects, but it will be interesting to see if Google can benefit from the potential of complementarity between its products that Facebook doesn't have. In my mind, two of Google's most valuable assets are Chrome and its affiliation with Android, both of which have been gaining marketshare. If they get a non-invasive Chrome toolbar and a sleek Android app out, you could get Google+ in front of a whole lot of users pretty quickly and make it a natural extension of people's mobile and browsing activity.

Also, Circles might help mitigate the network effect advantage of Facebook. Even if only one of your real-life circles (say your early adopting tech nerd friends) sign up at first, it can provide some functionality while it is still being more widely adopted by other groups.