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. At current prices Pinterest, Meta, Twitter, Alphabet and Snap were collectively set to lose about $42 billion in market value.