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.