People who install malware themselves are always gonna be susceptible to security issues. Even a hypothetically 100% secure OS (such a thing doesn't exist in reality, of course) would get infected if it was used by someone who downloaded everything they saw on some dodgy site.
Yes, social networking and ad agencies have a lot of security issues, but OS X does not get affected by anything which installs itself without the users' knowledge through browser exploits. This is because you also need to obtain at least admin access on the target machine.
Now, if a smart hacker who knew what s/he was doing was targeting you directly, then odds are, on a standard OS X set up, they'd eventually get in, but that's very rare for normal home users and has never, in the history of OS X, happened through a browser exploit. Fact.
Basically, your standard user will be fine, because, as any real geek will tell you, the most secure OS is OpenBSD, and OS X is based on BSD, giving it a secure core. The famous security researcher, Charlie Miller, even said that OS X Lion is the most secure consumer OS today.
