according to this press release from Warringah Council the Dam is getting around 1000 riders/week!

Media Release - Upgrade for Manly Dam
Date: Tuesday, December 9, 2014
Release: Major Upgrade for Sydney’s Busiest Mountain Bike Track

A treat for Mountain Bike riders is in store with the completion of a major upgrade of the Manly Dam Mountain Bike Track.

“The Manly Dam track has been working hard, with well over 50,000 uses a year, so it was in need of some loving care” said Warringah Mayor, Michael Regan.