You need to make a feature that no/few other fanfic sites have, because otherwise not many people will want to post on your site when there are more established sites like fanfiction.net or wattpad. You can do as much promoting as possible through social networking sites, but unless you have a really great site, that's not going to matter.

Understand your audience and where they are, maybe on other writing forums or fan websites, and direct people to your social media activity. Have a look at similar websites and work out what they are doing that works and copy those ideas in your own way.

USe your social media---FAcebook, Twitter, Pinterest, etc. Start by tweeting, posting and letting everyone know. On Twitter, follow everyone they suggest to you so you can broaden your outreach. On FAcebook, create a "page" for your site. Post things that you find interesting about it--and encourage people to check it out and "like" it. It takes a while, but it will eventually work.

use social networking sites to market urself and install pluigns for fb google+ on ur site

I've worked very hard to get this site up and working, however ... I don't know how to promote it and there are not much people knowing/joining ...

I guess I need advise in how to advertise:

Here is my site:

http://writefanfics.com/

or maybe you can tell me what you feel about the site.