Oh, you mean 50% of people that ever opened facebook have clicked on some ad at least once? Then it's not a very useful number - so suppose I clicked on some ad 5 years ago and since then never even logged in to FB - what use is to count that? I'd assume a useful number would be the number of active users regularly clicking on ads... IMHO 50% would be very high number then (unless you define regularly as "at least once in 5 years" :)