If you over-use it, it's not effective but for copywriting purposes, you'll get more clicks. Same with email: even if the subject is not a question, put a ? in

If you over-use it, it's not effective but for copywriting purposes, you'll get more clicks. Same with email: even if the subject is not a question, put a ? in it, and you'll get more clicks (it's been A/B tested by a few folks)