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!