Today Inkitt.com launched their Mystery / Thriller writing contest, called “Fated Paradox”.
See the details on the contest page: http://inkitt.com/fatedparadox
It’s for non published writers, but excerpts and old stories are eligible too, many established authors use their contests to promote their novels.
Their most active users include literary professors, and published authors too. They are proud of the high number of professional reviews and mature stories on the platform.