Every possible outlet.
I personally use a few different web things. I’m a regular user of Twitter, Facebook, Dopplr and LinkedIn. Many businesses these days have blogs, especially tech companies. Increasingly trendy web comapnies have Twitter accounts.
How do you decide which services to use and which not to use. In the past I’ve used Zopa, Shelfari, Tripwolf, MeetUp, HudDub and Squidoo. I’ve left all of these which varying levels of success. Often I’ve had to e-mail a real live person to actually get my account deleted, which is a hassle for me, and must be for them as well.
I’ve got a few questions about these services:
Is it worth the time? Do you really get the functionality you need? Does Facebook do anything e-mail doesn’t do already? I and a friend are thinking of starting our own little Web 2.0 company. Which services should it hook into?

