For reference, Pan Am has a roundtrip from PSM to SFB.. Clipper 1/2. Operates every day, except Tuesday.
It's interesting to note that Clipper Connection flights (3XXX-series) have actually increased from PSM lately. I believe that those Intra-Maryland flights (BWI-wherever) have been a bust, however.
Last I checked, there were around 10 72's sitting at the maintenance facility in Portsmouth. Around 5 were in Pan Am color's, and the remaining 4 were in Untied colors. The other one was an Air Miami jet.
I'm not sure how Newark-Liberty is doing for them, but I would bet that it's probably one of their better routes (PSM-EWR).