So you go to a site from google SERPS. Because google is sometimes a little careless the site may be less than trustworthy - in fact it's a virus planter.
Hey, guys, there's this +1 button - isn't that a google thing? Click - OMG a virus!
It can happen for FB as well - a well-known source of viruses anyway. As someone pointed out: FB has a recognisable logo that by now is well-known. It can still be a virus source, though.
Excuse me if I click on NONE of these links, especially on a site that's unknown to me.