How to keep my web accounts safe from spam and intruders?

Hi,
I am have an account with one of the social networking site where recently I receive complaints from my friends that they are getting abuse mails and postings on their page by me. I was shocked to hear this since it was more than a month I have logged into my account on the social networking site .
But somehow I can understand that it is being accessed by someone and its been used to spam and do all illegal stuffs. Am much worried about this where this is the most serious issue. So please anyone here help me with some suggestions to save my account from intruders and also provide me some tips on how to protect online entities.
