Never written to them and don't know how effective this is but I found this in the United FAQ's:

"Tip #7: Snail mail address for letters of commendation or complaint about United Airlines employees:

Ms. Sara A. Fields
SVP People
United Airlines
1200 E. Algonquin Rd.
Elk Grove Township, IL 60007

At least one FTer has reported that emails sent to UA (via 1KVoice or another email address) about employees may not ever make it to WHQ or to employees' personnel files, so if you want to be sure something arrives in United's HR department, lick a stamp."