When we talk about an end-of-life piece of software being a danger, it is not about your privacy, necessarily. These companies legally and overtly collect data about you and your browsing habits and use it to market to you more effectively. They do not depend on outdated software nor security vulnerabilities for them to do this. It is in their end user license agreement.I do FB, CL, Ebay and don't get paranoid over what any one of them does with my info. If people are so concerned over who is watching over their shoulders, they should throw away their computers and cell phones not to mention credit cards and bank accounts and live like it's the 19th century.
When we talk about security issues, however, it goes much deeper than that. The concern at that point is that your machine will be used by a malicious party for distributed denial of service attacks, etc. Some malicious parties may use vulnerabilities in your machine to use your machine for the transference of contraband data. The only limit is their own imaginations. And security software will often not pick these things up because the malicious party may not have placed an executable file on your machine that would run as a program. Beyond that, a malicious party may be able to steal information about you from your machine that will have a much more palpable effect on your life than a marketing organization being able to market to you more acutely.