New users to the site declined by 33 per cent year-over-year last quarter, according to Tech Crunch. Last month the site had 381 million total visits, down from 521 the previous year, according to Similarweb.
But Tumblr has reportedly faced a difficult five years competing against other social media platforms, including Facebook, Instagram and Twitter. Reddit has also continued to surge in popularity.
Reports indicate that Verizon had been shopping for a buyer for Tumblr, which hosts some 475 million blogs, since May.
Verizon acquired Tumblr through its 2017 purchase of Yahoo.
Yahoo had bought Tumblr for US$1.1b in 2013 but ended up writing off much of its value.
Reports indicate the latest purchase may have been for as little as US$3 million ($4.6m). Official terms of Verizon's sale to WordPress owner Automattic weren't disclosed.
"Today's announcement is the culmination of a thoughtful, thorough and strategic process," Verizon Media CEO Guru Gowrappan said in a statement.
"Tumblr is a marquee brand that has started movements, allowed for true identities to blossom and become home to many creative communities and fandoms."
Verizon had hoped to create an ad business to compete with Google and Facebook, but its media business ran into trouble.
It has cut jobs and sold some Yahoo properties, including the photo-sharing site Flickr and Polyvore, a fashion-focused "mood board" site that was then shut down.
Automattic has no plans to reintroduce adult content to the Tumblr site.