Facebook did blow up in select circles though right from the get-go. When it hit college campuses it spread like wildfire as there were no restrictions to entering and you could have all your close, daily contacts together in one place instantly. Google+ let me and 5 other friends in but prevented me from inviting others and now it's basically dead with the last post between any of us over a month old.
The overall number of users in the system isn't something important, it's how many users I know. A network with 10,000 users at Harvard will spread out much faster than one with 10m dotted around the US as it represents a contiguous userbase that is a solid foundation for social interaction.