By Echo Wang (Reuters) – U.S. stocks ended lower on Friday as shares in social media and ad tech firms lost ground in the wake of earnings from Snap, offsetting gains from card issuer American Express following an upbeat forecast. Still, all three major indexes posted weekly gains despite Friday’s losses. Snap Inc’s shares tumbled, after the Snapchat owner posted its weakest-ever quarterly sales growth as a public company, while Twitter Inc reversed earlier losses following a surprise fall in revenue. Online ad giants Meta Platforms Inc and Alphabet Inc tumbled weighing on the Nasdaq. Meta and…