Last time I used Facebook there wasn't a system of badges, karma, minimum karma levels required to access core features, and a community wiki for my wall posts. These are all highly nonobvious.
Embedded assumption #475 in Stack Overflow users: it is obvious that replying to a question is not Answering it.
I use StackOverflow. I've posted questions and I've read lots of responses.
I have no idea what the badges, karma, etc. are. For me, it works quite well. In fact, the only mildly confusing thing for me was that I had to use some OpenID thing to login the first time.
Think about that. Their value prop to someone coming in through Google or posting a question is simple and the barrier to entry is very low.
I've also posted questions and read responses on SO, but I believe I still haven't figured out how to "use" StackOverflow.
My first experiences with the site weren't too good: I tried to upvote a comment (I mean, there's a big fat arrow to the left of the comment!), and then I tried to answer the question and I believe it didn't let me either...
Don't forget the FAQ that demands you acknowledge its existence. I'm fickle enough that the top bar alone drives me away half the time. I'm not alone on this.
No, you're not. I remember looking at stackoverflow once and there was some sort of New here? Read this wall of text to get started! to which I responded Awesome! Maybe I'll read that when I have looked at the latest pictures on reddit!
Stackoverflow incorrectly assumed that I gave a shit about their website. They can get away with it because they have technical users who often don't react like that. But if your users aren't technical, then you can't possibly overestimate how much they don't care about your website.
* Posting a question: I go to the site, click on "Ask," and post my question.
* Searching for answers: I go to the site, type in my search, and search.
Facebook:
* Posting a photo: I go to the site and look for a camera icon. I can't find one, so I start clicking on stuff like "Account", "Home" and "Profile". Still nothing. I start clicking on everything that isn't a friend's wall post. Eventually I hit "More" on my list of apps (the fully-expanded list still doesn't take up the entire height of my screen), and see "Photos". I look for an upload button and see nothing. In the upper right corner, in 10pt font, I eventually find "My Albums." I click there and search for an upload button. Nothing. I click "Create album," and finally get to an upload prompt.
* Changing a privacy setting: I see a post on Hacker News about some new Facebook feature, and of course, the default is "Everyone in the world can see everything that this feature involves, even if it's something you locked down for every other feature." No one has posted a reply explaining how to lock it down yet, but it's Facebook, right? How hard could it be? I go to Facebook and click "Privacy Settings." Honestly, they change the layout every few months, so I have no idea what it looks like now, and I don't remember what it looked like last time, but every single time, I have to re-learn the entire model and it takes forever and half the time I don't even find what I'm looking for.
* Take a quiz: See a quiz on a friend's wall. Click on the app name. Click on "Take Quiz." Get a popup saying "Let this app access your profile: [Click Here]." Close tab.
People don't come to facebook to post photos. The right thing to compare posting photos on facebook would be, maybe editing your user profile in stackoverflow.
Also, posting on stackoverflow requires you to be familiar with Markdown, an obscure markup language based on conventions used by geeks on mailing lists.
On facebook, the first thing you do is look at other people's posts and statuses. And that's incredibly easy; you don't even do anything, facebook just shows it to you.
It's incredibly easy for you because you know how facebook works.
The first time I used facebook, I was like whaa? What's a WALL? What's the difference between a post and a note? How is sharing a link specifically different from embedding it in your status?
Lots of complexity there, but we learn it after a while.
People join facebook because all their friends are on it, and they get to see a news feed.
It's not instantly familiar or usable, and no one knows what's a wall or a news feed, but when you find pictures of a friend you haven't seen in 10 years, you'll probably stick around and try to figure it out.
Of course, at the beginning the only users where college students and people in their teens or twenties. But now even people in their forties join just because they can keep in touch with their distant relatives and old friends.
I suspect it's a reference to the structure of the community, reputation system and permissions that result. The way those three interplay is pretty complex - how you incentivise certain behaviour, unintended side effects and so on. Some of the more interesting bits on the SO blog are Jeff's comments on what they've done, where it's gone wrong and how they've tried to correct it.
On that sense Facebook is very simple as what you can do is almost entirely driven by whether you're friends with someone or not, and the mechanism for becoming friends (or stopping being friends) is straight forward.
Obviously that's a somewhat simplistic view though and ignores little factors such as doing even the simple things with 300 million users is massively complex.
From a content standpoint, StackOverflow requires the participation of domain experts to be successful. Facebook just requires people to sign up and log in.
That doesn't make it more complex, it just means that it has a very specific requirement for the knowledge of the participants.
From a software perspective it's really no harder to gather and store 1000 words on a detailed C++ problem than it is to gather and store "Mmm mmm mmm, Justin Bieber, he's soooo dreamy".
