It is difficult to say right now. As a private company, much information is hidden from plain view. In debt, Facebook has received approx 15B in commitments that have some sort of debt terms behind them that would require servicing. Clarity is not known there. Facebook's infrastructure and human capital needs are growing on a daily basis as well. In 2008 (a long time ago in internet years), it was widely reported that FB broke even with roughly around $300MM in revenues. In today's numbers, expect this to be nearly double ($500MM) given the increased expeditures in infrastructure and people as the site has grown exponentially.
Assuming this VERY rough imaginary cost, a 750MM membership base, Facebook would need to convert 1% (7.5MM) of the user base into premium users at a cost of $5.56 per month to break even.