It's almost impossible to sign up for an account with Twitter, Google, or Facebook without divulging your phone number with them and then confirming it. They're at the stage where their growth is scrutinized by investors and partners, and they're desperate to prove that their users have real eyeballs that can see ads. Before that, they had no problem with new accounts maybe being bots because it's good for growth numbers. Now advertisers want to know if real people are seeing their ads, or if just bots are.