How can I make my site more social / involved for posters?

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.

1. Make it real : The experience of the site needs to add real value to the people who visit. they really -- especially in the beginning -- need to receive immediate validation of the time they have spent.
2. Make it fake : Seed the site. Work with a group of friends or beta users to really seed the site with the type of valuable comments that you want on the site. I know I said "make it fake" but that was really just to be overly cute in contrast to the "make it real" comment. the comments and contributions that you and your friends overload the site with in the initial period of time need to be real too. They might be a little extra intentional, and sometimes forced -- but the visitors to the site will experience the authenticity of meaningful comments.
3. Communicate : Communicate regularly with your initial users. eNewsletter and such will be great as the community grows. Right now be personal. Send personal emails. Send personal Facebook messages. Pick up the phone and call them. I strongly recommend you integrate a Live customer support platform onto the site and engage people in real time when they are on your site.
4. Build the Community Leaders : In a beta and initial roll-out the most valuable asset you have are the folks who take a sense of pride and ownership in the site. These are the folks who will visit regularly, encourage others, and nuture the development of the community. They are not appointed, elected, or hired -- they emerge. As they emerge -- lift them up!

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.