Radio had its first down year since 2001 as total revenue slipped 2% last year to $21.310 billion. Off-air dollars were the biggest gainer, increasing 10% to $1.7 billion. Those dollars, which include things like website and mobile phone advertising, now account for 8% of radio’s revenue. But it’s radio’s staple spot sales that were the weakest. Local ad revenues fell 2% last year. National was off 6%.