The trickiest part about Facebook's IPO is deciding when to buy some shares. Consider what happened after last year's IPO of LinkedIn, an online professional networking service that is probably the closest thing Wall Street has seen to Facebook's social network.LinkedIn's shares rocketed from $45 in its IPO pricing to $122.70 within the first few hours of trading. A year later, the stock hasn't touched that price again. But a month after the IPO, patient investors were able to snap up LinkedIn's shares for under $64. The stock has bounced back above $100, now that LinkedIn has proved it can be more profitable than analysts anticipated. Skeptics believe LinkedIn is grossly overvalued, just as the doubters are harrumphing about Facebook. Both companies are expensive by traditional benchmarks. LinkedIn has a market value of about 12 times its projected revenue this year, while Facebook went public with a market value of about 20 its projected 2012 revenue. Google, by comparison, has a market value of about six times its projected revenue for this year. But Facebook hasn't been as aggressive as it could have been about selling ads or finding other ways to make money where its visitors, on average, dwell for an average of 6Â½ hours per month, according to comScore Inc. Instead of ramping up revenue, Facebook has concentrated on attracting users â¿¿ an emphasis that is bound to pay off. One of the main reasons Facebook is likely to figure this all out is that Zuckerberg hired Sheryl Sandberg as the company's chief operating officer in 2008. Sandberg played a key role in expanding Google's advertising system during its first few years as a publicly held company, a period when the company's stock hit its peak so far. Sandberg brought not only her own expertise to Facebook but also hundreds of other former Google employees who defected to the social network in search of the next big thing.