Twitter last week purchased Yes and named the mobile app startup’s CEO, Keith Coleman, as its new VP of product — a challenging post. Coleman will be Twitter’s third VP of product this year and the fifth since 2014. Twitter’s main problems are monetization, which has plagued it since its founding; slowing user growth; and stiff competition. Those encouraged by Coleman’s appointment cite his previous experience at Google. He served there as the project manager leading Google Plus, which includes Google Reader and Google Talk.