Facebook revenue growth slows, costs weigh on profit (Reuters) - Facebook Inc posted its slowest growth in quarterly revenue in two years and higher spending on research and development ate into profits. Shares of the world's largest Internet social network fell 2 percent in after-market trading on Wednesday. Facebook has warned of heavy investments in 2015 as it steps up efforts to expand a collection of products that include messaging service WhatsApp, photo-sharing service Instagram and virtual reality headset maker Oculus Rift. As a result, Facebook's operating expenses rose 83 percent in the first quarter as R&D costs jumped 133 percent and marketing and sales spending nearly doubled. However, Facebook said monthly active users grew 13 percent over a year ago to 1.44 billion, with 87 percent of them accessing its service on smartphones and other mobile devices. "It's a generally solid quarter.