"Use tweet button very smartly" -- elaborate ?
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.
Another test we ran on our Facebook page (http://facebook.com/pinwoot) was to have a tab called "Try Now" which was basically connected to our Facebook connect app - that was great, many many conversions. So, we are still using it.