Billboards are so last season. Now, when it comes to a lot of advertising, it's all about Instagram. Or, more specifically, Instagram followers. Models are no longer cast purely on their vital statistics - their social media statistics also have a bearing on their bookability."For certain big money jobs, brands stipulate that girls must have at least 10000 followers," Select Model Management's Paul Hodgson said recently. ''Almost every brief we receive now asks how many followers they have."It's easy to see just why, then, that models such as Cara Delevingne and Kendall Jenner - who have 13.9million and 28.7million Instagram followers, respectively - keep scoring top-notch modelling gigs. The higher the number of followers, the greater the exposure for the brand.Enter Katy Perry, the latest celebrity-model hybrid to flex her social media muscle in order to boost a campaign.Unveiled as the first celebrity face of Moschino last week, the singer has been drip-feeding further campaign images to her 22million Instagram followers via her 71.1million Twitter followers and whipping up a whole load of publicity in the process."MORNING! For the next five hours I will post a new KATY x MOSCHINO picture through Instagram each hour, on the hour," she informed her followers on Monday, before doing just that.This clever use of Perry's almost-100 million-strong social media following means Moschino reaches far more people far more quickly than by placing an ad on a billboard in Times Square, which even at its very busiest sees 480000 people pass through it in a day. So is the billboard obsolete? Looks like it...