Last time I tried to sign up for Twitter it demanded I verify my account with text messages. Actually, virtually all services do this now when creating an account. The worst (Microsoft for example) let you sign up and use the account for a bit (possibly purchasing some items tied to the account) and then extort the phone number out of you later to maintain access.

It is sort of amusing that these companies hitched their wagon to the now scam laden telephone network to track users and ended up getting scammed themselves.