Can you "Facebook" your membership?

Facebook has just hit 250 million users -just three months after hitting 200 million users. This is one of the most fabulous examples of membership growth I know of. Here is an organisation that understood a need in the marketplace, developed a membership product that fulfilled that need and experienced enormous growth as a result. They went from 0 members in 2003 to 250 million members in 2009. Even better - they have a free membership structure that is supporting from other income sources that leverage off their strength in numbers. With vision the sky is the limit. How can you "Facebook" your membership?