norton full system scan sufficient checking for all viruses?

[Stevo]

I had just done a full system scan, disk clean up and disk defragmentation on my computer. I have norton security had it had fixed all threats during the scan and a day later my email and facebook got hacked. I only knew the passwords and always sign out of my accounts. I'm wondering if norton full system scan is sufficient for checking all viruses and if anyone knows how I got hacked or how to prevent this from happening again.

Thanks for your help!