>But I wonder what their clickthrough-rate is.
It's shockingly high, especially on Facebook Audience Network (FAN), Facebook's network of 3rd party publishers.
It was so high that I initially thought I was being defrauded. But upon closer inspection, it turns out that if you specify your industry/persona well in your ads, Facebook is incredibly efficient at matching demand with supply. I've never seen such good performance on any other mobile network, Google included.
FWIW, I've only done B2B marketing in my life. I have no idea for B2C folks.