I don't think Reddit will ever scale as well as something like Facebook . The bigger Reddit gets, the less usable it becomes due to subs becoming too crowded.
The whole point of reddit is that the crowding issue is self-correcting -- you just move to a new sub. I like to envision it as a malthusian catastrophe -- as the population starts reaching capacity limits, people start grumbling more and more.. until suddenly they far overshoot the capacity, and leave en-masse to new subs. In one fell swoop, the original sub is left near-empty, and out of the many new subs created in that instant, a few survive with healthy populations.
I have been on reddit a long time and I have never seen a popular sub ever die unless it gets banned by admins. Splinter subs emerge but the original one remains popular too
The long-tail subs will never get too crowded. If you have found a niche where like-minded people hang out, there will never be a whole lot more of them, and however many more there are, it's a positive.
