Social networking sites

Questions:

• Are there any issues I should be aware of, to safeguard myself when using social networking sites such as Facebook/Twitter?
• Someone is posting false information about me on Facebook/Twitter/a social networking site. Is this a criminal offence?
• Someone is sending me threatening / abusive / offensive messages via a social networking site. Is this a criminal offence?
• What is cyberbullying?
• What is 'Zoom bombing'?