I've just created a new site which allows users to post content around their local area and any activities.
As it has just been launched there is not much in the way of content for those people who are going to come and browse the site.
What I would like is to make it more involved for people who are coming to post the content.
I have a good rate of registration; about 10% of people who are visiting the site. However, these people are dropping off once they make a single contribution.
How can I make the site more involved for this group of people and make it so they would want to come back?
from your description I think you can look at what meetup is doing - weekly emails on activities around the area, joining an activity and seeing participation lists, specific interest-based groups etc...
on the social side I like integrating facebook comments besides the usual tweet and like buttons.
As tommyng noted, weekly email reminders work quite well.
Add an "invite your friends" feature, where the user can import his friends from gmail/linkedin/facebook and select the ones they want to invite.
Give special "prizes" (can be as simple as badges, or real prizes if you can) to people whose invited friends actually subscribe to the website.
you need a niche grabber to put it simply. The idea of a Social network that doesnt have a focus is a flop from the word Go
Start controversy... anything that will stir the pot.