By Richa Naidu LONDON (Reuters) – Stephanie Labarta, 31, was six months pregnant when she first noticed the occasional infant formula ad on Instagram and Facebook. Now, a year later, she sees ads three or four times a day, typically for Reckitt Benckiser’s Enfamil or Nestle’s Gerber Good Start. “Gerber actually came up this morning on my feed – it started off with a contest to submit your smiling baby and then it trickled into the different types of formulas and what they have available,” said Labarta, a senior analyst at a nonprofit in New York. Soon after the influx of social media ads began…