Spotify has brought its ‘welcome back’ ads, which greet returning users, to the mobile sphere. Since the ads are immediately served up to users upon return, Spotify says that they guarantee advertisers “100 per cent viewability.” The ‘Overlay Mobile’ ads are an extension of Spotify’s ‘Billboard’ desktop ads, which are essentially large display ads that appear in the center of a user’s screen once they return from a period of inactivity.