From my experience of social media if you don't provide a phone number when signing up for an account, then after a few days/weeks they will lock your account due to 'suspicious activity' and the only way to get your account unlocked is to provide a mobile number for them to verify it.
The whole suspicious activity is obviously a complete lie and just a way for them to force you to provide them with a mobile number they can then use to track you better or you loose access to the account.
Biting the hand that feeds IT
