By Gerry Shih SAN FRANCISCO (Reuters) – Facebook Incs investors and other proponents of the social network like to say that it captures one of the greatest concentrations of human attention on the planet and thus offers a boundless opportunity for advertisers. But Facebook Chief Financial Officer David Ebersman on Wednesday cast doubt on those assertions by suggesting that there may be a limit on how many ads Facebook can show users before they get turned off. Ebersmans warning carries far-reaching implications for not only Facebook but also other social media companies including Twitter Inc, which is in the middle of a roadshow to promote its initial public offering to investors. Twitter has yet to turn a profit but it is pitching an advertising business model similar to Facebooks. Its important for investors to realize that there is a limitation on the mobile ad revenue that can be generated.