TBG Digital, a Facebook advertising agency, estimates 93 million of Facebook's 900 million users have mobile accounts. So, Facebook's market may be 10% lower than it appears. Facebook estimates half of users access via mobile phones, so half of users may not see ads as they are not displayed on the phones currently. Then, there's the issue of user saturation. Facebook has done a great job in penetrating key demographics in key geographies, that is, countries with an established consumer economy. But, it may have already saturated critical segments. In the U.S. Facebook has 157 million users and 90% are over 18. About 91 million are ages 18-35, the target demographic for advertisers. Census data puts 75 million people in the 18-35 category. So, if you do the math, Facebook has 120% penetration of the 18-35 segment. Jerry Thomas at Decision Analyst, an advertising researcher, estimates 60 million US residents aged 18-35 have PCs. So, using Thomas' data, Facebook has 50% more users than the segment has PCs.