As all those social services (Facebook, Twitter, Google Plus) like to know where users surf. All the websites with like buttons create a log entry in their logs and include a reference to the profile, if you are logged in. This way they can track you surfing over the web.
One thing I do myself for a long time is to delete cookies automatically. With Cookies app from Russell Gray. I have 20 cookies on my white list like the Xojo and FileMaker forums auto login. But all others cookies are automatically deleted when I close the browser. This way I'm a new client each time when I visit google web search or browse amazon.
Now I have several browsers: Safari, Firefox and Google Chrome. For my normal surfing I use Safari while the social networks reside in Chrome. They can set their cookies and I have auto login, but they don't monitor me when I use Safari to surf the web.
Finally I disabled plugins. No flash security problems if Flash is disabled. And as YouTube now uses HTML5 by default, that's not even a problem for normal surfing. And of course plugins are just a few clicks away if needed. But with new plugin per website control in modern browsers, you should have plugins disabled for all websites by default. Only enable them where you need. It is amazing how many websites want to use Flash only to track you!