Object 2011 MD is thought to be between about 30 to 100 feet wide. If it had "hit" the earth, it would most likely have burned up in the atmosphere with no fragments making it to the surface. Asteroids similar in size to 2011 MD make such close approaches to earth about once every six years or so on average, but screwing up those odds, another rock flew by within 3500 miles of earth just this past February.
The main thing to keep in mind is we had less than one week's warning to the presence of 2011 MD and we possess zero current technology or capability to do anything to ward off such a threat.
The six person ISS crew was ordered briefly to take shelter in two docked Soyuz capsules in response to the near pass of the space junk. If a threat comes with more warning than this most recent one, NASA has the option to make slight orbit adjustments to keep the ISS clear. The origin of this particular object is unknown.
The takeaway in this case is that largely through our own ignorance and sloppiness, low earth orbit space is relatively teeming with potentially harmful space junk of widely varying size, origin, and description. And this problem is going to become worse before it becomes better.
“Watch the skies. Keep looking. Keep watching the skies!”The Thing From Another World (1951)