LOS ANGELES — When it comes to choosing the right medicine for their kids, moms go by the book — Facebook, that is. Moms new and old are spending more and more time on mommy blog-type sites and then tweeting or “Facebooking” any interesting tidbits they may find there. It’s a highly engaged group of consumers. According to recent findings by market researcher Morpace, U.S. Facebook users are on the site for one of every three minutes of time they spend online.