Social network and Web2 apps

Popular social networks and Web2 applications:
Twitter - Flavour of the month.
Best for keeping up-to-date your idols, a particular topic (eg. #surfing) and/or members of your team.

Linkedin - How timely.
If you think that there is a chance that you might be unemployed soon; hop on this network (don't look too desperate). Then if the crunch comes you might be able to use your presence here to get a job or at least to use it as a reference.

Blogger / MySpace / Multiply - DIY websites or a scrapbook.
All those people you call friends are by definition interested in what you do. Put a collection of your stuff somewhere where they can see it. I prefer Multiply for this purpose.

Facebook - "Friend collecting"
As an older person I use it to keep tabs (in a nice way) on my relatives in that Facebook age group. It helps that I am not the slightest bit judgemental.

It is by playing with tools that you learn how to use them.