By Sarah McBride and Poornima Gupta SAN FRANCISCO (Reuters) – At Pinterest, the online bulletin board service that is valued near $3.8 billion, some 70 percent of the users are female. Male-heavy boards dominate in the start-up mecca of Silicon Valley, which prides itself on progressive thinking and putting talent first. A Reuters survey of the 10 top venture-backed start-ups, as measured by venture funds raised, shows that six do not have any women on the board, including Pinterest. Reuters research relied on publicly available data and discussions with start-up executives and board members.