By Noel Randewich
April 28 (Reuters) -
Shares of both Pinterest Inc and Snap Inc tumbled about 18% on Friday after the two social media companies' quarterly reports spooked investors worried about weak digital ad spending.
Their disappointing reports late on Thursday stood in contrast to strong results earlier this week from digital advertising heavyweights Alphabet Inc and Meta Platforms Inc, and show that advertisers are sticking with the biggest players in an uncertain economy.
Pinterest forecast second-quarter revenue growth below Wall Street estimates late on Thursday as the image-sharing platform grapples with a pullback in advertising spending in a market it warned remained uncertain.
Also reporting late on Thursday, Snapchat owner Snap missed analysts' expectations for revenue, blaming changes to its advertising platform that it said hurt demand for ads.

It warned results in the next quarter could fall below Wall Street's targets.
Snap said it was taking steps to improve the relevance of ads shown to users, and those changes in some cases meant advertisers saw fewer "actions," such as users' tapping on ads.
"Aside from Snap´s internal issues, the competitive landscape remains daunting, and we believe the darkest days of this downturn are ahead of us," Monness Crespi Hardt analyst Brian White wrote in a client note on Friday.
The two companies lost a combined $6 billion in stock market value, with Pinterest now valued at $15 billion and Snap at $13 billion.
Short sellers betting against Pinterest and Snap were up about $240 million in mark-to-market profits due to Friday's selloff, Ihor Dusaniwsky, stars77 managing director of predictive analytics at S3 Partners, said in an email.
At least 12 analysts cut their price targets for Snap, while Pinterest saw at least seven price target cuts on its stock.
Shares of Meta Platforms have surged 13% since Wednesday, when the owner of Facebook and Instagram forecast quarterly revenue well above analyst expectations.
Also on Wednesday Google-owner Alphabet's quarterly results showed ad sales held up better than expected.
(Reporting by Noel Randewich; editing by Jonathan Oatis and Leslie Adler)