(In my mind, only markets tend to be winner-take all. are there other examples?)

Winner Takes All is more of a continuum than an absolute.

For example, most people say that a social network site is a winner take all market and that FaceBook has it.

While they have the majority, Linkedin also has some share, Google+ has some share, even MySpace still has some share. All the competitors still have huge valuations relative to anything most people will ever do.

Further, Facebook doesn't have anywhere close to a monopoly on internet advertising dollars let alone all advertising dollars. There are many companies that earn money through "eyeballs".

Basically the Winner Take All Markets concept is an oversimplification, but still a great way to think about competition.

I definitely agree. It's useful as a way to analyze a competitive space, not an absolute.

Social networks act like them sometimes. If you get everyone using your network, it becomes so valuable that its almost impossible for a competitor to break in.

definitely.

(actually, of the four types of companies i look at as an angel, my two favorite are "markets" and "networks")

What are the other two?

"products" and "content"