Investors who managed to buy Facebook Inc shares ahead of the No.1 social network\'s initial public offering might want to think twice before \"flipping\" them.Brokerages can impose restrictions on participating in future IPOs for those trying to profit from market debuts. Some serial flippers could even face lifetime bans. Strong demand for the third-largest initial share sale in US history has fueled anticipation of a big pop in Facebook\'s stock price once it begins trading on Friday. Only Visa Inc and General Motors Co had bigger IPOs. While investors are free to sell the shares they picked up via an IPO whenever they want, many brokerages have policies in place aimed at discouraging the practice of selling them too soon, because holding onto the shares post-IPO supports the issuer and the stock. Main Street-focused E*Trade Financial, along with TD Ameritrade, and Charles Schwab Corp, define \"flipping\" as buying shares of an IPO and selling them within 30 days from the date the IPO goes public. E*Trade and TD Ameritrade reserve the right to exclude customers who flip shares from participating in future IPOs, according to their websites. Schwab clients who flip shares from a public offering are restricted from participating in initial and secondary public offerings through Schwab for 90 days, said spokesman Michael Cianfrocca. At Fidelity Investments, customers can sell after 15 days without raising any red flags. \"But if they sell within 15 days, they\'ll be prevented in participating in another IPO for 180 days,\" said spokesman Steve Austin. If clients buy a second IPO and sell that offering before the 15-day period, they\'ll be prevented from buying an IPO through the firm for an entire year. The third time triggers a lifetime ban on IPO deals through Fidelity, Austin said.