That still allows geolocation. But one can lease hosted SIMs. Actual SIMs in server farms. So they're "real" mobile numbers. Just not located where you are.
I doubt twitter is going to get your cell tower location just from your phone number. Sure, the cell provider has it and is probably keeping it around, but unless you attract significant attention to yourself (eg. bomb threat), it's not going to be disclosed. If you're really paranoid you can use it a few blocks away from your usual location.
The ToS of those providers prohibit the use of that except with explicit consent. I'm aware that the providers do little verification that consent was given, but I'm doubt that established companies such as facebook or twitter are going to risk a PR fiasco to get better anti-fraud numbers.
