You need to send friend requests first to people who you have mutual friends with. This way Facebook and the User sees you as a trustworthy account. If you continue to mass-send out friend requests to a bunch of randoms, then you will be flagged and kept on a close watch.
I suggest going through your current friends list and find their friends with the higher number of mutual friends and send out requests slowly.
I also found that when "accepting friend requests" once your profile gets big and popular and your getting hammered with friend requests make it look real and decline some. Just bulk accepting got one of my accounts blocked once.
Starting out I found you really have to start slow. Do not expect to create an account and have thousands of friends in a few days. Grow them steadily and I have never had a problem.