Look at the lunar surface, it’s completely covered with evidence of massive impacts ....We too have been hit many times in the past by large objects, but since our surface changes, it hides or covers up many of these impacts.
They’re also small enough that most of them don’t make a crater on the land. :’)
http://www.freerepublic.com/focus/bloggers/1250694/posts
But it’s not unheard of, either:
http://www.freerepublic.com/focus/chat/1216757/posts?page=3#3