: Since a social network targets users online, you should build a strong online presence for your new site: pitch big online magazines. If you manage to get covered by Mashable or Gigaom, you'll

Since a social network targets users online, you should build a strong online presence for your new site:

pitch big online magazines. If you manage to get covered by Mashable
or Gigaom, you'll get your first users instantly. Search for the
submit news form (http://mashable.com/submit/) or contact journalists
directly via e-mail/Twitter/Google+, etc.
create some viral content - a video can be a good choice. This should
be something really catchy and humorous and it should highlight
the features of your network.
introduce the system of invites - something which is limited at the
beginning can look much more attractive than when it's available
to everybody.
be ready to answer the main question - why should I register on one
more social network and spend even more time online?
try any kind of online advertising to get maximum online visibility
(Google AdWords ads and banners, paid placements on popular blogs,
etc.)

Whatever you start doing to promote your social network, be sure to give it some time - there's almost no way of getting instant success - unless Mr. Zuckerberg himself announces the launch of it :)
Good luck!