Since late 2016, Facebook Inc investors have increasingly debated the sustainability of Facebook's advertising and overall revenue growth over the medium term. But, UBS is bullish on the company's prospects to increase its ad revenues in the medium term.Analyst Eric Sheridan expects Facebook to increase its ad revenues at about 28 percent over the next three years, driven primarily by Instagram. Sheridan’s channel checks suggest other key drivers being video and eCommerce.Facebook remains among the best positioned companies to capitalize on the continued shift in ad spending from TV to digital platforms.“Specifically, we believe the secular shift towards mobile advertising, growth in users & engagement, and ad innovations (video Ads, Dynamic Ads, etc.) will drive FB's ad revenues over the next few years,” Sheridan wrote in a note.As a result, the analyst expects video ads revenue to grow significantly over the next few years, accounting for about 18 percent of total advertising revenues in 2019.via