In my opinion, Psychforums.com is by far the best place to post online studies, but your school/university needs to establish a partnership with them first. Best is to speak with your professor or someone responsible for your Psychology department in order to have that partnership in place, which is in fact really easy (see the post before). Once done, all the students/researchers/professors of your school will be able to post there, so it's not on a per-study basis. Also, your study can be exposed in 3 specific forums.
The problem with the other places is that it's focusing on online studies, so only people searching specifically for online studies will found them, and you also only get participants interested in cash reward, which I suspect change results, as it drags in people that might not be interested in your online study at all. On Psychforums you can have very specific targeted audience as well as a very general audience depending on which forum you ask to be exposed to, and those participants are not coming to Psychforums initially to participate in a study. Of course it depends on your study, but in my personal experience, I got more than 50 participants in a week, while all the other places I've posted brought me less than 5 participants.