I'm creating a forecast for my free membership-based website. I want to create both a membership growth forecast and a revenue forecast.
While every website is different, I'd like to know how various popular sites like Facebook, LinkedIn, Bebo, etc. grew, the tactics they used to grow, what caused the viral growth, etc.
Ideally I'd like to know user growth by quarter or month from inception.
You might be asking the wrong question because:
Notice that a lot of the answers here involve leveraging existing social networks like Facebook, Twitter, and I would add LinkedIn and mailbox integration with GMail, Yahoo Mail, and HotMail.
Of course when Facebook was growing, these didn't exist (in the massive, ubiquitous form they're currently in) and many saw them as a MySpace competitor.
Point is: However Facebook grew, the customer landscape is now very different, and therefore it's wrong to rely on their numbers or tactics.
(+1 on your question anyway, cause it is a good question!)
One of the problems with trying to use other company/people success is that there is a bias towards "survivors" and there may not be any usefulness in copying them.
Essentially you can look at the outcome of facebook and other popular sites as the one lucky survivor out of thousands of initial participants. Eventually you will have a company that has "made it" - however there may be nothing useful to emulate and it might have been sheer dumb luck.
An example from a scam that may clarify what I mean:
One well-known scam is to mail/email to a list of, say 2^N participants, predictions for the outcome of a sports event. (1/2 will get one result, while the other gets the other result) Included with the prediction is a note saying that for some dollar amount we will keep giving you our amazingly accurate predictions. Successive predictions are then sent to the subset (likely half) of people who received the "correct" predictions in the past.
The receiver of the information only sees successful predictions. (until they get the one that isn't) Thus they are amazed at how well you can predict the outcomes of some events.
The most powerful element in growing a social network is referrals via your new user's contact list.
The ability to import address books automatically to send out invitations is key to viral growth.
This tactic is so crucial that Facebook acquired a company for such talents recently.
Imagine if there is an average of 100 addresses in each new user sign up and 10% actually signing up upon receiving the email invitation - if your site started with 100 new users a day, you would be sending out 10,000 emails a day, with 10% of that, 1000 signing up (which would in turn introduce 100,000 emails going out, with another 10% of that signing up, ad infinitum).
Eventually your site would reach some sort of an equilibrium - but that's how sites with sticky and good enough features that implement address book importing gain extra momentum with such a tactic.
Along with Alex's address book integration, I would include other social apps. I just saw a Facebook post that indicated it came from Twitter. I've seen wireless posts as well. You don't want to get dropped by users because your app is just one more thing to worry about.
Don't forget email (As much as some of us would like to.). Not as full proof as some think (See Coding Horror ), but many users cling to it.