Facebook’s more than 2 billion users make the social network practically indispensable, but even Facebook relied on growth hacks in the early days to rapidly expand its user base. There are a few tried-and-true methods for expanding your Facebook following, as revealed by this Quora question:
Inviting users to include their personal networks
If a person is mentioned or tagged on Facebook, an email will be sent to that person’s contacts.
People’s curiosity was piqued by the mention notification emails, and they signed up for Facebook to check out the conversation that had been started about them.
Source: optinmonster.com