Mataf.net's Arnaud has compiled an interesting comparison chart of various Forex forums and portals. He uses Google adplanner to estimate the number of unique visitors to each site and while this is not the most accurate tool of them all this is still something that can be used as a reference.
It seems that forexfactory is by far the leading site in terms of traffic followed by dailyfx which turned out to be a stunning success by FXCM - I'm sure it brings more clients to FXCM than any other type of direct advertisement. Another interesting finding is that all the social sites altogether (meetfx, currensee, myfxbook, etc) are still very small in terms of traffic. I think that Currensee's numbers are way below than what they really are but as I said, it's still an indication. It will be interesting to see the results of same comparison a year from now, I believe social sites will grow tremendously in the coming months.
I'd also like to note that 'biggest' sites are not the most profitable sites - CPM can significantly vary between sites and if I were to structure such a comparison using income as a vertical axis the results would have been different than what you see below.
Read more here.