To get the individual user information for a Twilio number, given the short duration they might be attached to a given account, I highly suspect you have to go through Twilio for that info. That is, Twilio is likely to be the sole source for information on which account is attached to what number at a point in time. Obviously any consequential government authority in the US can then get that from Twilio, assuming the required judicial merchandise is presented.

I'd be curious though to know if eg Twitter security sends a request over to Twilio (perhaps in a case of an account abuse investigation), if they'd cough up information very willingly. That I don't know. It's a certainty the government can get at it, however. The best bet is to do nothing illegal using a Twilio number, and to assume if a big external service provider (Twitter, Facebook, et al.) needs to get details for some legal reason, they will.