One of trending 'dirty' trick while signup is multiple social account signup. Signup form shows social singup like linkedin or facebook. User chooses one, say linkedin, you think it almost signedup. Then application would say something like - we 've stopped linkedin sign up - use facebook instead. You are left no choice to use facebook. But if you look closely, they use both social account graphs to learn MUCH more about you. Latest big name I came across doing this is slideshare!

If you list you age between 12 - 14, tgey will let you sign up without providing any other information.

Haven't been able to replicate this.

I was able to register an account without a phone number using this method.

I generated a ton of google accounts this way during my Pokemon Go phase. Maybe it doesn't work anymore?