Malware (short for "malicious software") refers to programs designed to gain unauthorized access to a computer system, to destroy data, and to change the whole working of the system. Viruses fit into malware. They can be transmitted via the Internet including through e-mails. Other type of malware include Trojan horses, computer worms, and spyware.
Facebook and other SNS are targets for malware. The advertisements on the site cannot really be trusted as spyware can get installed on one's computer. Recently, in Japan, one company's system was attacked by virus. It came via an e-mail from someone who posed as the legitimate sender.