SAN FRANCISCO (USA TODAY) — Facebook on Wednesday filed to go public and raise $5 billion in what could be the largest-ever Internet IPO.

Once Facebook stock starts trading under the symbol FB, which could happen as soon as May, the onus is on the company to ramp up revenue to maintain its lofty valuation. Facebook's fortunes hinge on advertising, which accounted for 85% of its revenue last year, or about $3.15 billion. Social-games-maker Zynga accounted for 12% of Facebook revenue. Google raked in $37.9 billion last year, 96% from ads.

The good news is Facebook commands a 28% chunk of the U.S. online-display ad market, up from 21% a year ago, says market researcher ComScore

I definitely see the Facebook is on the downfall now. Every social networking website has/had its time. Earlier it was Orkut, then its Facebook. However, in the times of competition, LinkedIn still stands apart. It does not have any competition as yet!