People who aren't good with computers (and who are not interested enough in their more esoteric functions to learn more about them) often use Google (because a Google searchbar it is right at the top of Internet Explorer's window) to find what they want rather than type the URL into the URL bar or learn to bookmark it. They don't know what bookmarking is, and all those colons, acronyms, and slashes in the URL bar make it seem to complicated to bother with. It's not necessarily a matter of stupidity, it's a matter of convenience for someone who takes web-surfing as a very low priority. If you already know how, typing the URL or bookmarking the site IS faster than googling it. If you don't, then googling it is much faster than taking the time to learn the alternatives.
Since Facebook is just a way to keep in touch with friends and do other "real life" stuff, it attracts a lot of people who would otherwise hardly ever turn on their computers at all. It seems silly to us because almost everyone on this forum is an internet addict.
Except that these people seem to completely lack the common sense to see that a website with "ReadWriteWeb" in huge letters, without any mention of facebook beyond the article, and completely lacking in anything resembling a "Login to facebook" option might just possibly be a website that isn't facebook. It's like if you go to a newsagent to get your regular magazine from its usual spot, only you see that it has a different title, is a different size, has a completely different layout, and is about dog breeding as opposed to cars. Would you assume you'd picked up a different magazine and go look for your regular one, or would you go shout at the newsagent and/or write angry letters to the publisher because your car magazine is suddenly about dog-breeding?