outsources to Air Wisconsin, ACA
(soon to end), Skywest, Shuttle America, Mesa, Chautaugua, and Transstates. This is the largest collection of any U.S. airline.
US outsources to Mesa, Air Midwest (owned by Mesa), Chautauqua, Colgan, Shuttle America (soon to end), and Transstates. This is the second largest collection of any U.S. airline. (It's worth noting that US also uses wholly owned subsidiaries PSA and Piedmont as well as pseudo-espress wholly owned carrier MidAmerica). This is quite a collection of outsourced carriers also.
The two majors in the worst financial shape have the longest list of express carriers by a long shot. Is there a causal link? I'm not sure, but it is interesting to note this pattern.