1 The founders populated the site with Questions
In Quora's early days Adam D'Angelo and Charlie Cheever wrote so many questions to fill their site with the needed data.
2 Early Employees filled the site with Questions
According to a thread on Quora early employees kept doing the same thing the founders made which is filling the site with questions.
3 Adam D'Angelo had good connections
Adam D'Angelo, Quora's founder, had many good connections because he was the CTO of Facebook before starting Quora. Those Connections helped Adam get funding, media attention, and great employees.
4 Adam D'Angelo had good money
Adam had good money because he was a Key employee at Facebook. Adam spent a lot of money on Quora. It was reported that he invested $20 million of his own money in one round.
5 Adam : We invited our friends
On one Quora thread Adam said that they invited their friends who also invited their friends and so traction happened.
6 A Tech community was built
Quora focused on tech questions in the early days. This attracted smart Tech geeks who are smart and dedicated. This kind of community helped the site have high-quality intellectual answers. (See Why is Quora better than Yahoo Answers?)
7 Beta invitations targeted the right people
According to a Quora thread beta invitations were sent to people who are well connected and who have good knowledge. This helped the site build traction.
8 Quora got big traffic from Google
In one interview Adam said that Quora focused on the topics that can't be easily found on Google such as information on startups. This strategy helped Quora rank high on Google which led to massive Google traffic.
9 Focusing on one topic brought more Google traffic
Google ranks websites based on their authority. Quora started with so many answers about Tech questions. This resulted in many back-links coming to the site and so Google considered it an authoritative site in the tech field.
10 Paid traffic
It's widely believed that the founders paid some money to get some initial traffic. (See How Quora started?)