Two answers to this one: sort of, and absolutely.

Sort of means you can go to these sites and select to deactivate your account.  The problem is that while it is deactivated, all your information, history, friends lists, postings, tweets, etc. are all maintained.  You can reactivate almost any time you want.

Absolutely means actual removal of all traces you existed.  This is provided free by The Suicide Machine!  It takes several days, but it is very very thorough!

Enjoy!