Captcha for Twitter, Facebook and the like

The idea is to keep the bots out, right? Just one obligatory captcha per month if you want to post something at a random day and swoosh the net is clean again. However these captchas need to be updated regularly and a time limit would be a good idea. Fail to solve the captcha and you're in for more. Might be a good idea even for steemit. You don't want no bots messing with your crypto, right?