Most of Reddit's user base is from Digg, you could say that as soon as Digg's user base started to build, Reddit's did.
You make it sound like social networking has always ruled the internet, but the line between the internet and social networking was blurred only recently, and wasn't blurred during the creation of thefacebook, and Digg, and arguably wasn't blurred during their rises to fame.
I also haven't heard of much links from Reddit being posted onto Facebook nor seen it at all. In my experience most link sharing for Reddit is done between friends in all sorts of context, which may include but isn't limited to Facebook.
I can understand having closed networks of friends on the internet, I enjoy the convenience of this every day, but to not have my real name plastered all over the internet is an even bigger blessing. Anonymity is powerful on the internet because instead of your identity & reputation coming from your name, they come from your idea that you put on the internet.
People are biased to be more agreeable and acceptable of ideas coming from people they respect (ex. Steve Jobs), and they don't tend to think about the entire spectrum of said idea if it comes from that respected person. In other words, they are more biased on average to think about the good of the idea rather then the bad. This means that they are judging the idea based on the person, rather then the idea based on the idea.
The problem with an open social network that envelopes the entirety of the web (Facebook is trying to do this), is that my choice of participating in society by being on Facebook and communicating on a daily basis will all my friends suddenly becomes a very dangerous choice. Now, not only am I selling out all the information that I pass on to my friends, I am now selling out (almost) everything I do on the internet, whether or not I'm logged into Facebook or not.
On a more extreme side are websites that force you to log into Facebook Connect to use their site, so they can gather more information about you. My choice to participate with my large network of friends becomes even more dangerous, as I now have to give out even more information to more companies to use most of the features that said Facebook Connect site has.
Not only that, but the ideas I post on said site are suddenly linked to my name forever, so some employer may deny me a job now because I think that Linux is overrated, or the iPad is a good product, or Steve Balmer is a shitty CEO.
---[This is a long specific example, you can just skip down to the dotted lines if you hate reading and hate bunnies.]---
Let's take this one step further though, so say I'm at some bar and I meet some chick named Betty that I think is really interesting and cool, and we exchange full names so we can find each other on Facebook. Everything is looking real good, I mean really good, we're going to see that new awesome comedy that is coming out about that guy who gets cancer.
But wait! As it turns out, Betty decided to go check out what I've posted on the internet by searching "(Full Name)", and she sees all these sites that I've posted on that have forced me to use Facebook Connect. Well, as it turns out I think extreme feminists are absolute lunatics who are more about female rights then equal rights, and it also turns out that I like to get wasted on Saturday nights when I'm surfing the internet, so what she finds is me shitting all over some bat shit crusader on (some Facebook Connect site), and I'm cursing all around being a dick. (Everyone is a dick some of the time, the internet just makes this eternal)
So she finds this post of me being a dick, misreads into it, and decides I'm some chauvinistic bastard, and then cancels the date raging about me publicly on Facebook. (Like I said before about being a dick). Well, I'm fucked out of a date, and if some guy searches up about Betty and sees that she raged about some dude publicly over the internet he might cancel that date.
---[Start reading again here if you hate reading and hate bunnies.]---
Now, this example is really shitting into the wind, but everything on the internet is recorded, cached, and we don't have access to delete everything on it. People will misread into that information, and other people will buyout and abuse this information despite what a policy may say (if there even is a policy). Even worse is that Facebook 'deactivates' your account at your request but keeps all the info on you to monetize. This isn't limited to Facebook either, just go look at Google Analytics and how useful Google Plus is to it. This doesn't even touch onto the possibility of Facebook 'interfacing' with the mafia, all someone has to do is pay the price that Facebook names for that information.