Well, I don't know about later models, but on my 011 EL, I can say without hesitation that although the bumpers are plastic/rubber/fiberglass on the outside, they are very solid, hard-assed US-made steel underneath.... ask me how I know....
A) Cause my son (6' 5"/375lbs) and me (5' 11"/240 lbs) can stand on the rear bumper at the same time, nottaproblemo, and
B) Thanks to a mild frontal run-in with a guardrail a few weeks ago, there was alot more actual damage to the guard rail than to the bumper itself, save for the rub marks & scratches and a small crack on the outside
@rockeye
Unless you somehow managed to drill through the mounting points for the steel part underneath your bumper (unlikely), then the part that was coming off was most likely the plastic/fiberglass part that covers the actual bumper.... what year is your Expy ?