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 had a problem with tons of bots signing up for my Nintendo site so I put a single image of Mario on the sign-up page (making sure nothing in the image data said "Mario") with the text "Who is this? Answer in one word." Haven't had a single bot sign-up since. Not sure if this is actually a good solution though, not sure how smart bots are. I'm kind of surprised that it worked.

In theory it might be keeping out a few legitimate users, but it is hard to imagine many legitimate users of a Nintendo site not knowing who Mario is...