Mobile social networks will experience a steady growth curve, but making money on them will remain difficult, according to a report published by In-Stat. It estimates that by 2012, there will be about 30 million “millennials,” or gen-yers in the U.S. using a mobile social network. Internationally, that number will soar from 525 million this year to 975 million in 2012, reports ComputerWorld. But the big problem is not in adoption, but in finding a way to make money. In-Stat concluded that the three primary ways to generate revenue on mobile social networks is advertising, subscription services and premium upgrades. Still, with those options available, In-Stat estimates that advertising revenue in the U.S. will only total $1.5 billion this year, increasing by less than 50 percent to $2.2 billion in 2012.