We have to engage/activate our existing users to get more users. For that twitter is the best mode.
On signup, we request users to tweet (just a request) - about 10% new users to do this tweet. The language of this tweet was selected after number of (sort of) A/B testing with different texts.
In the dashboard, when ever a user gets more than 10 new followers, we start showing a message that provokes them to share a message like "Woot! I got X #Pinterest followers from Pinwoot for free. If you are on Pinterest, you must try it". This tweet is resulting in 15% of new signups daily.