Cryptocurrency peddlers are being let back into valuable social-media advertising space, after Facebook Inc. eased an outright ban on ads from the industry.

The tech company put a blanket ban on such ads in January, after a spike in the price of bitcoin and other cryptocurrencies led to a proliferation of new cryptocurrency products—some legitimate, others scams. Companies including Alphabet Inc.’s Google, Twitter Inc. and Snap Inc.’s Snapchat soon followed with their own bans.

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