What is this? This is where a membership waiting list can be skipped via social sharing. Users are freely able to sign up but upon signing up a user enters a waiting list (sometimes their waiting list number is show, sometimes it is not) and the user is encouraged to “bump” their place up in the queue by inviting their friends or sharing the page via social media.
Why should I do this? The waiting list itself can lend a sense of exclusivity to your product. The additional queue jumping via social sharing mechanic adds a layer of promotion on top of this. Your product gets exposed to the social networks of anyone who wants to jump the queue.
Who else is doing this? The most famous example was Mailbox, who employed a waiting list system to prevent too many people signing up at once since the product was still in beta. Users would sign up and see that they were the (for example) 153,280th person in the queue, but would have the option to bump themselves up the queue if they shared the app on social media. Many users opted to do this, and as a result Mailbox was widely spread throughout Twitter soon after launching. Mailbox never actually publicly launched and were acquired by Dropbox while still in beta for a reported $100 million.
How can I implement this? The waiting list functionality itself could be achieved with a cron job that simply runs every hour or day and updates a small batch of users to an upgraded status. As with The Human Pyramid, the social sharing component will need Twitter / Facebook API integration to function properly. A user would click a button which would post to their social accounts and they would be moved up the queue in your database.
One caveat with any kind of social sharing mechanic is that the user can simply delete any content posted from your system, after they receive the benefit (such as jumping the queue). This negates the point of asking them to share. While it is technically possible to check for this behavior via social APIs and penalize users who do this, it is an added layer of complexity for what may for you just be a minor edge case.