Most of them desperately need talented people, so they have to advertise their openings aggressively. But the more jobs they post, the more they expose themselves to people like us, who know how to analyze the postings. If you examine the kinds of backgrounds that a company looks for in its systems people, you can get a good sense of its technical infrastructure. Say a search unearths a Web site that's really valuable. It's a nice feature.
Search engines are a mystery.
A few months ago, a colleague of mine did a training class on using the Internet. The same colleague did a search that morning and got a bunch of hits. She did the same search in the afternoon and came up empty. Home pages are such an obvious resource that people often don't take them seriously. It's a great place to gather intelligence on that company.