Apparently at least one major government had an insider working undercover at Twitter:
https://www.nytimes.com/2018/10/20/us/politics/saudi-image-campaign-twitter.html
It doesn't really matter if Twitter is trustworthy or not, simply having a vast amount of personal data in one place is going to attract spies from all sorts of organisations that wish to misuse it.
This is why decentralisation is important, to make it technically more difficult to invade people's privacy.