title: How websites like Facebook are protected against bot without any captcha
body: <p>How websites like Facebook and Twitter are protected against bot during registration? I mean, there's no captcha at all on the signup form?</p>
  
  <p>I want to create a signup form for a project, and I don't want bot during registration and Captchas are often ugly..</p>
  
  <p><strong>edit:</strong>
  My question is really during the registration because I know Facebook uses Captchas once registred for the first time.</p>