Open Brainstorms

Send members weekly updates. In each update give members an "invite friends" option.

facebook made their social networking site exclusive to Harvard, and then Ivy League Schools, and then State Schools and then Tech Schools, and then International Schools, and then high schools, and then EVERYBODY. Sometimes you have to start with a more exclusive audience and build a buzz

make sure people can find it by googleing mallorca city--since there is a real city you aren't on the first page

make the site look cool, innovative, more color

Have annoying TV commercials like Zwinky.com. Someone will eventually be curious and see what it's about. Get Zwinky, Get Zwinky---it sticks in your head.

Give people something of value if they recruit ten of their friends to join

Does your site have a niche? Perhaps it should be specialized and advertised to a specific group of people

Open up your source code for developers to develop exciting applications for your website.

Start a contest of some sort on your site with a prize based on how many people you invite to your network and give a prize to the "leader" every month. Display the leaderboard publicly. People who want to prove their influence should be invited manually to "seed" the system.

Do TV advertising for cheap using http://www.SpotRunner.com

Announce your social network on sites like Emily Chang's eHub

Post your site URL on craigslist, facebook, myspace and other sites which already have good viewership.