twitter, the WYSIWYAD (what you see is what you are doing) platform - not to confuse with twitter - incorporates a social network structure. Any user can have other users marked as friends. A simple visualization of all direct friends of a randomly chosen twitter user and their relation to each other could look like this:
If we look one recursion level deeper into this social structure it might divulge an even more complex structure: johl-2-1.pdf
The software used to extract these graphs from twitter is available from my projects page.