Social proof accelerator: Airbnb makes sure you do tell your peeps about the service by giving you a fair share. 

Get free stuff: Stickermule’s way to get new customers and reward loyalty. They’re giving you and your referral 10$. Nothing new, but i like it. 

Social proof: Fancy Hands offers 50% off for people who have followed an endorsement. Don’t fear to make that social proof very real.

First come first serve: The Macupdate Bundle includes a special treat for early buyers. A significant choice when you consider bundles like this may profit a lot from social proof.

Encourage sharing: @Gidsynews lets you know how many spots are left after booking an activity so you can immediately invite your friends to join in. 

Realtime social proof: The Humble Bundle (@humble) used a real time bundle sales counter where you could tell that the product sales like hot cakes (along with a time indication to let you know how long you had left to profit).

User registration: Friend.ly uses facebook data to make it easy to sign up with prefilled forms whenever you open the site in your browser. All you need to do is adding your current location.

Additionally it displays how many of your friends already signed up (social proof).

Social Proof: Honestly (@honestlyreviews) sends a welcome email telling you which of your (facebook) friends are already on the service. 

LinkedIn (@LinkedIn) wants you to notify your contacts whenever you update to a paid account (if you don’t pay attention, they will).

Use of social proof to handle beta invites: Turntable.fm makes it’s app inherently social from the beginning (playing music with friends works best).

Update: They have just added a new social feature; when you like a DJ you can follow him and receive an email whenever he starts playing in a room.

Update II: You might also like to read How has Turntable.fm grown so rapidly with no marketing?