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

I don't know how Facebook and Twitter do it, but if you want to create something simple and that doesn't interfere with your site aesthetics, I know that some websites just ask the user to enter an answer to a simple math problem like "what is 2 + 3?". This is not the most secure way to do it, but it's just a thought.