First time I posted a job on oDesk it was for a Mac desktop developer. I got 4 applicants, 2 of whom I had invited to apply based on a search. I knew this one would have too few applicants.
Second time I posted for a Community Manager and got less than 20 applicants in a week.
Third time I posted for a Windows desktop developer and got about 20 applicants in two days.
Today I posted for a web front end developer and got 25 applicants in two hours.
You may draw your own conclusions!