Just to correct one of Kenny's points, there is no "flow back" for mainline UA pilots, as there is no flow. Some regionals, like CommutAir, allow pilots that leave to come back within 3 months and keep their seniority, but as every UA Express carrier is its own entity, there is no provision for mainline UA to displace Express pilots.
American is a different kettle of fish, as they do have a dedicated flow program, which works in both directions if necessary.
Commutair is still recruiting and expanding, albeit at a slower pace.