It may just as well be that the number of competing job boards doubles ever year and the postings distribute equally. Reminds me of TV channels: they kept increasing until eventually no one cared anymore and cut cable all together and became more selective again about what they subscribe to (netflix, hulu, hbo, etc.). With this constant increase in job boards, they may all become collective irrelevant eventually and maybe people are indeed no longer posting jobs on them. So maybe both of these effects are at play? a job-boards bubble?