People like to share and recommend services. It makes them look knowledgeable, advanced, and "in the know." Make it easy in your product for users to refer their colleagues and friends with one click - send the email for them - even pre-write it.

Do the same with Twitter - make it easy to tweet from within your product. Pre-write a tweet and use Bit.ly to track the link

Don't do this too early in the user experience - make sure this only shows up after you can conclusively say a particular user has received value from the product. Asking for a referral too early can only annoy users and has little chance of being used.