If you've ever tried to figure out what a "social graph" really is as a term of art (it has to do with a branch of mathematics called graph theory) and how it's actually different from what we've all been calling "social networks," Dave Winer helpfully explains this for you:
Graphs are useful for modeling stuff that goes on in computers. They are also part of a field of math called combinatorics that's related to statistics, and also related to a highly theoretical area of math called topology...If you're a specialist in any field of endeavor, you can probably come up with many examples of how a "term of art" in your field has a subtly--or radically--different meaning as a word in common English usage.
...[B]efore we talked about social graphs we called them social networks, and you know what -- they're exactly the same thing, and social network is a much less confusing term, so why don't we just stick with it? (Answer: we should, imho.) So if you don't want to sound like an idiot, call a social graph a social network and stand up for your right to understand technology, and make the techies actually do some useful stuff instead of making simple stuff sound complicated.
If you're trying to communicate with people, here's a big hint: Use the word--or the meaning--that they're likely to understand.
How to avoid sounding like a monkey (Dave Winer, Scripting News, 21 September 2007)