Thailand is prosecuting Facebook, Google, and Twitter over a failure to remove what it says are illegal posts.
Officials did not say exactly what they contained but also announced they would take action against individual users for insulting the monarchy.
Under Thailand's strict "lese-majeste" laws, doing so can result in a prison sentence.
This is the first time that computer-crime laws have been used to target the platforms themselves.
The announcement follows mass protests in the country demanding political reform.
"We've notified the companies and sent them warnings twice, but they haven't complied with all the requests," digital minister Puttipong Punnakanta told Reuters news agency.
He said the matter had now been referred to cyber-crime police for prosecution.
At issue were 661 Facebook posts, of which the company removed 225; 69 on Twitter, of which five were removed, and 289 YouTube links, which were removed on Wednesday after the deadline, according to the Bangkok Post.
Under the computer-crimes law, the social-media companies can be fined 200,000 baht (£4,970) for ignoring a court order to take down posts - and an extra 5,000 baht every day until it is removed.
In August, Facebook blocked access to a one million-strong Facebook group in Thailand that discussed the monarchy, after a threat of legal action from the Thai government.
At the time, Facebook told the BBC it was preparing its own legal action in response to pressure from authorities there.
The accompanying announcement about action against individual users follows an anti-government protest last week.
Only a handful of prominent accounts are believed to be involved in that.
The protest in Bangkok involved thousands of people calling for reform of the country's political system - including the role the monarchy plays.
Police estimated about 15,000 people took part; organisers claimed 50,000 did so.
The calls for reform of the monarchy are highly sensitive. Thailand's criminal code allows for prosecution for criticism of the royal family - often in secret trials, resulting in long prison sentences.
Human rights groups allege that the law has been used as a way of limiting free speech and calls for reform.
Facebook and Twitter declined to comment on the case, and Google has yet to respond.