Chinese online marketplace Temu and fast-fashion retailer Shein, two of the biggest advertisers on US social media, are sharply cutting their US digital ad spending, industry data show, in a blow to tech companies such as Meta’s Facebook and YouTube. The online retailers, both of which ship low-priced China-made goods direct to US shoppers, had been on an ad spree until recently, targeting younger, thriftier shoppers in digital media.