You bet your ass they can. Since when has Facebook taken anybody's privacy seriously? And you remember all the Snowden leaks? Like how AT&T has been a government apparatus for spying for decades? Or how about the way that the USA taps under sea cables to monitor data, causing China to build totally parallel backbone infrastructure
The better question is whether Signal, despite being open source, is actually secure. It's very plausible that the govt has backdoors somewhere, for either encryption, the OS, the programming language, the app store, or some random dependency lib
The answer is yes, the US government spies on everything, and has a complete profile of everyone