I think the latter is more likely. If you stay in physics then you are more likely to be a poster on the physics forums. If somebody switches fields they are more likely to be at the forums for that field. Thats why this forum's regulars seem to be professional physicists and physics students.
I think this forum generally has the more optimism about physics grad prospects than anywhere else I frequent online or in real life. Even the pessimism about being a "physicist" in this thread is tempered by the claim that physics grads still do better than average. Considering my graduating class, they are not in science for the most part but they are doing better than average, career wise. They were smart hard working people before the physics degree, they still are smart hard working people and its not surprising they excel in their non-science careers.
People who left the field are probably not here but at Wilmott or some other fields forum or a some hobby forum like woodcrafting forum.
People here are supposed to have enough training in probability and expectation values to understand that given the AIP , BLS data and the poll by ZZ the highest likelihood outcome is to not end up working in physics (How many more data sets do people need to see before everyone agrees that the highest likelihood outcome is to not stay in physics).
I would also hope people have enough training in the sciences to understand their personal anecdotal experience doesnt over ride the much bigger data set of the AIP or BLS.
And no this does not mean if you do physics you will most likely end up homeless because that isnt the case for any major. Even if you didnt graduate high school you still dont have a greater than 50% of ending up homeless so everyone can hold back on the hyperbole.