At a recent company meeting, management told LinkedIn employees they would soon be allowed to sell as much as 20 percent of their vested options at a $500 million valuation. Word leaked yesterday that Facebook plans to allow its employees to do the same . Both LinkedIn founder Reid Hoffman and Facebook founder Mark Zuckerberg want to take their companies public — and thereby get their employees paid — but it won't happen soon. LinkedIn expects to earn about $100 million in 2008, but VentureBeat reports that bankers want to see that number hit $200 million before bothering to file papers. The public markets aren't hungry enough for anything less. In July, only 56 companies went public , raising $5.6 billion in their IPOs. During the same month last year, 190 companies raised $31.7 billion on their initial foray into the public markets.