You could do several classes around Groupon and LivingSocial's product / product marketing expertise. Here is a very simple example of how Groupon does the little things very well.
Many sites have Facebook Connect buttons across their header or footer - and when a user is logged in, their name and Facebook icon appears. Like many social features, Groupon goes a step further. In addition the large Facebook icon, the promotional module features similar content to the new Facebook widgets: the number of your friends who are also connected and the names / images of a few of them. For a popular service like Groupon (154 friends are connected!), this is a compelling way to welcome and encourage users to connect.
Once you are connected, an invite button appears. When clicked, it allows you to share directly on Facebook... and it automatically includes your Groupon referral link. Referrals are an important marketing / viral mechanism for sites like Groupon, Gilt Group, etc - and while they each have their own invite flows, this is a clever, streamlined integration.