This site calls itself ‘the most comprehensive people search engine on the web’ – it crawls what it calls ‘the deep web’…
“Also known as “invisible web”, the term “deep web” refers to a vast
repository of underlying content, such as documents in online databases
that general-purpose web crawlers cannot reach. The deep web content is estimated at 500 times that of the surface web, yet has remained mostly untapped due to the limitations of traditional search engines.
most personal profiles, public records and other people-related
documents are stored in databases and not on static web pages, most of
the higher-quality information about people is simply “invisible” to a
regular search engine.”
I’ve tested this out and you will find that while it does find a lot more info than say a Google search would and you should be able to find most people in the developed world you may not be able to access a whole lot of information on those people. In any case, if you looking to track old acquaintances give this a try first.