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If you've spent time on Tumblr, odds are you've encountered porn. A study from 2016 showed that nearly half of its users end up seeing porn on the microblogging site. But starting today, that's going to change.

Tumblr's ban of all adult content takes effect on Monday, Dec. 17. Tumblr, one of the oldest blogging platforms, defines adult content as "photos, videos, or GIFs that show real-life human genitals or female-presenting nipples, and any content -- including photos, videos, GIFs and illustrations -- that depicts sex acts."

Tumblr will allow nudity in some scenarios. Breastfeeding and nudity in art are still okay to post.

Still, some Tumblr users have strong opinions about the site's new policy (and a handful of false categorizations), and are posting their reactions to other social media sites.

"We knew this wasn't going to be an easy task and we appreciate your patience as we work through the challenges and limitations of correctly flagging tens of billions of GIFs, videos, and photos," the company said in a post Monday. "This is a complex problem, and over the coming weeks we will gradually, and carefully, flag more adult content."

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The service, which hosts about 450 million blogs, stressed that it would be hiding, not deleting, content that's been flagged as adult. Once flagged, the post will be set as private and only be viewable to the poster.

Tumblr CEO Jeff D'Onofrio published a blog post on Dec. 3 describing the thought that went into the policy change. Despite the new policy, D'Onofrio recognizes that "Tumblr is also a place to speak freely about topics like art, sex positivity, your relationships, your sexuality, and your personal journey."

He said he hopes the new policy strikes a balance between allowing self-expression and creating a safe place for everyone.

First published Dec. 3 at 11 a.m. PT.

Update, Dec. 17 at 10:06 a.m. PT: Added new comment from Tumblr as ban on adult content goes into effect.

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