Digital publisher BuzzFeed is on track to miss its revenue target this year by a significant amount, the latest sign that troubles in the online-ad business are making it tough for new-media upstarts to live up to lofty expectations.

BuzzFeed, one of several digital-media outfits that have grown quickly in recent years, had been targeting revenue of around $350 million in 2017 but is expected to fall short of that figure by about 15% to 20%, people familiar with the matter said. BuzzFeed said in a statement that it would still...