Apartment List is fighting "the network effect." In its case,
that means Craigslist.  When an online service gains a critical
mass of interdependent customers, it stops making sense for any
of them to use another service. Facebook is perhaps the best
example. The central reason people use Facebook is that most of
their friends are on it, which makes starting a rival social
network very difficult.  The same principle applies to
Craigslist. The Internet's largest collection of classifieds
indexes everything from couches to favors to jobs to apartments.
Yet Apartment List believes that it can make inroads. The
4-year-old SoMa company pulls in apartment data from various
sources and renders it all on a searchable map so you can browse
neighborhoods quickly.  The company did $10 million in revenue
last year, says chief executive John Kobs, all without the aid
of venture capital. He expects revenue to grow substantially
this year, but acknowledges that Craigslist's hold on the
apartment market will take some time