How do large companies protect themselves from spam?

Question

In contact with

Specifically interested in Vkontakte. What are the main methods of dealing with bots? It is known that Indians enter captcha for a couple of cents.

• Whether input speed, mouse behavior is analyzed
  
• What methods to discover the same device besides IP
  
• Is text analysis the main way to fight
  
• How will a real person's browser differ from, for example, mechanize or robobrowser,
  and indeed from pure request.get
  

Answer 1

protected by this akismet.com
this is protected by a machine landing
and the Indians are crying from Recaptcha2 (with pictures of which)

Answer 2

AJAX onSubmit: Token->T1(client)->[link]->T2(server)->Result
if (Result) -> [Token Confirmation Procedure]

if (T1<x*lyambda || (T2-T1)<y*lyambda) 
       return false; //not in time-range - go away!

For guests, the best for me is complex behavioral control (client + server).
PS: About the captcha (the one that everyone is used to) - it makes no sense for VK, because. there is activation by mob. phone. Blocked for spam a couple of times and brought.