July 22 (Reuters) - Shares of social media firms fell sharply on Friday as Twitter Inc TWTR.N joined the Snapchat owner SNAP.N in signaling a cutback in digital ad spend as economic growth sputters. Advertisers have pared back spending amid rising interest rates and surging inflation as some of them struggle with labor shortages and supply chain disruptions, Snap Inc said on Thursday. The Snapchat owner's weak quarterly outlook confirm fears that ad spending is worsening, RBC Capital Markets said in a note.