I found some numbers at
http://www.transtats.bts.gov/DL_SelectFields.asp?Table_ID=260&DB_Short_Name=Air Carriers (Bureau of Transportation Statistics) which looks at the number of passengers on international routes. I've pulled out Porters monthly transborder numbers for Jan-Sep 2010. The numbers shown are "to YTZ"/"from YTZ".
Jan 2010: BOS 1729/1858, EWR 5904/6100, MDW 1798/1722
Feb 2010: BOS 1495/1472, EWR 6397/6385, MDW 1934/2035
Mar 2010: BOS 2009/2029, EWR 8155/8167, MDW 2796/2805, MYR 410/444
Apr 2010: BOS 2949/3057, EWR 7064/7699, MDW 3351/3609, MYR 418/454, YYZ-PHL 59
May 2010: BOS 3423/3311, EWR 9243/9017, MDW 4537/4502, MYR 375/304, YUL-EWR 70
Jun 2010: BOS 3447/3693, EWR 8700/9164, MDW 4491/4691
Jul 2010: BOS 4392/4587, EWR 10295/10606, MDW 4946/5180, YQB-DET 36
Aug 2010: BOS 4923/4589, EWR 11599/11440, MDW 5410/5165
Sep 2010: BOS 4104/4143, EWR 10501/10731, MDW 4312/4415
We would need to work out how many flights there are per month to determine load factor, but those numbers seem quite respectable.
EDIT: For comparison, I dug out the ORD-YYZ data for the other airlines. I'm only going to show the ORD-->YYZ numbers in the format Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep:
United Air Lines Inc. 6421/7984/11751/8850/10736/7046/6424/6993/5959
Shuttle America Corp. 4054/3317/3445/3678/3592/3641/4082/4666/7145
Air Canada Regional 621/2710/406/427/743/632/409/317/500
American Eagle 8867/7968/11150/6219/6195/5956/6369/6596/6424
GoJet (United Express) x/531/1127/1408/1383/2333/2790/2467/108
Trans States Airlines x/33/x/265/351/437/335/270/x
Air Canada 6543/3832/8782/7755/8788/9278/x/x/x
Air Canada doesn't seem to have any numbers past June. Air Canada codeshares with GoJet, United Express, Trans States and Shuttle America, so the field isn't actually as crowded as it looks.