Apple on Thursday in a letter addressed to nonprofits, including Human Rights Watch and the Electronic Frontier Foundation, saying it doesn't hoover up detailed user data to sell to third parties. Facebook takes a "very different approach," Apple said.

"Not only do they allow the grouping of users into smaller segments, they use detailed data about online browsing activity to target ads," Apple's Senior Director of Global Privacy Jane Hovarth wrote in the letter.

"Facebook executives have made clear their intent is to collect as much data as possible across both first and third party products to develop and monetize detailed profiles of their users, and this disregard for user privacy continues to expand to include more of their products."

Facebook hit back hard in a statement sent to Business Insider, accusing Apple of abusing its dominance to benefit itself.

It also told that Apple sent the letter as a "distraction" from privacy concerns that emerged last week after a series of Mac computers had difficulties opening apps.

"The truth is Apple has expanded its business into advertising and through its upcoming iOS 14 changes is trying to move the free internet into paid apps and services where they profit," Facebook said.

"As a result, they are using their dominant market position to self-preference their own data collection while making it nearly impossible for their competitors to use the same data. They claim it's about privacy, but it's about profit," the statement read.

"This is all part of a transformation of Apple's business away from innovative hardware products to data-driven software and media."