FORTUNE -- Paul Adams is one of Silicon Valley's most wanted. He's an intellectually minded product designer with square-framed glasses, a thick Irish accent, and a cult following of passionate techies. As one of Google's lead social researchers, he helped dream up the big idea behind the company's new social network, Google+: those flexible circles that let you group friends easily under monikers like "real friends" or "college buddies." He never got to help bring his concept to consumers, though. In a master talent grab last December, Facebook lured him 10 miles east to Palo Alto to help design social advertisements. On his blog, Adams explained, "Google values technology, not social science."