Some speculate it was due to their blog post that went viral on Facebook Twitter Pinterest and Reddit - "A Complete Guide to finding a job on Facebook": http://www.glassdoor.com/blog/complete-guide-finding-job-facebook/

Others think it was:

1. Open Graph integration.

2. The language "To activate your connections, invite these people" and when you pressed the button it invited all of your friends.

3. More great language: "Friend X just activated a connection", "Friend X wants a favor from you", etc.

One last note: only 25% of new users were retained as of a week later. Exponential growth usually leads to exponential decay: http://pandawhale.com/convo/3484/facebook-open-graph-offers-exponential-growth-followed-by-exponential-decay

If you trick someone into signing up instead of acquiring users honestly by doing something useful for them, you always end up with app resentment.

And yet, most Facebook apps grow by bribery, spam, and porn.

"Tricking them in" probably works if you subsequently provide value. But you wouldn't need to trick people if you were promoting yourself well in the first place.

In general, it's better to avoid tricking people. No one likes to be tricked.

It's much better to make a great product and offer incentives to try it.

That makes users feel like they're discovering and using the product on their own terms.