At http://phys.org/print291396902.html … we learn that more than ten thousand asteroids and comets that can pass near the Earth have now been discovered. They range in size from a few feet across to 41km across. Asteroid 2013MZ5 is about 300m across – so fairly small on the average. In the 20th century the add up rate was small – just 500 by 1998. After that date, with the advent of super telescopes, that figure has escalated.
At www.dailygalaxy.com/my_weblog/2013/06/nasa-1000-near-earth-objects-large… … the same story but focuses on the fact that one thousand of them are larger than 1km across.