How websites like Facebook are protected against bot without any captcha

Question

How websites like Facebook and Twitter are protected against bot during registration? I mean, there\'s no captcha at all on the signup form?

I want to create a signu

Answer 1

One way would be to send a verification to the user's email address or cell phone and obtain verification (so in that case, you would have to allow only one email address or cell phone per account)

Another option is to use "Negative CAPTCHA" or "Honeypot Captcha"