Pinterest Inc PINS.N plunged 11.3%, Facebook-owner Meta Platforms Inc META.O dropped 5.6%, Google-owner Alphabet Inc GOOGL.O, which also sells ads online, fell 3.3%. By Medha Singh and Akash Sriram July 22 (Reuters) - Shares of social media firms fell sharply on Friday after Twitter Inc TWTR.N and Snapchat's owner SNAP.N signaled advertisers had tightened their purse strings in response to a darkening economic outlook. 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.