Step for a social site

Social Grow cycle.

Step 1
active user < 100
active network < 10

So sorry, your site not yet a soical site, please think about something non-social first.
(ie, write some article to let people read, or make some mini-game, or find some expert to answer question at your site)

Step 2
active user 100 - 1000
active network 10 - 100

Thing about "Challegne" which about number of "friends", to move people to invite friends.
Thing about some small game, which need more than 3 day to play (facebook present, it will grow within some day).

Step 3
active network 100 - 1000
and some of the netowrk size > 50 person
after this stage, total number of user is not very meaningful

Thing about how to make people find their friends within the site.
Call back the idle user, by their friend is joined or new join.

Step 4
active network > 1000

Think about open platform, let user to build the site for you.