Facebook, Google and Twitter suspended processing government requests for user data in Hong Kong, on Monday (July 6).
The moves come in response to China's establishment of a sweeping new national security law for Hong Kong.
And in an illustration of worries about the law, short-form video app TikTok announced -- it would be pulling out of the Hong Kong market.
Facebook, which also owns WhatsApp and Instagram, said in a statement that it was pausing reviews of all of its services "pending further assessment of the National Security Law."
Google, and Twitter said they suspended their reviews of data requests from Hong Kong authorities immediately after the law went into effect last week.
Although the social network declined to comment, Twitter cited quote "grave concerns" about the laws implications.
Google said it would continue reviewing Hong Kong goverment requests for removing user-generated content from its services, while Facebook did not respond to a request for comment.
Tech companies have long operated freely in Hong Kong, where internet access has been unaffected by the firewall imposed in mainland China, which blocks Google, Twitter and Facebook.
Social networks often apply localized restrictions to posts that violate local laws.
And in the second half of last year Facebook restricted nearly 400 such pieces of content in Hong Kong.
Adding to the Monday announcements -- TikTok, owned by China-based ByteDance, said it would pull out of Hong Kong within days.
A source told Reuters that the move was made because it was not clear if Hong Kong would now fall entirely under Beijing's jurisdiction in light of the new law.
Asked about the moves by the U.S. tech firms and prospects for media freedom, Hong Kong Chief Executive Carrie Lam said on Tuesday "the law would not undermine human rights and freedoms."
Since the law came into force some Hong Kongers have started reviewing or deleting social media posts they thought would be too sensitive.
On the streets many Hong Kongers say they're worried about online freedom of expression.
"Meanwhile the fear has already spread over Hong Kong about cyber expression, the freedom here."
Messaging app Signal, which promises end-to-end encryption, has seen a surge in sign ups in Hong Kong.