Quality over quantity is what should be aimed for. The internet evolves and social sites get to a point of imploding. For whatever reason, and people then move onto something else. Some of us can remember BBS and IRC.
Each place shouldn't set out to be the previous sites' replacement. It should take what worked, the good parts, and build on them. Mix them with something new, and experiment. This way, you are not directly competing with the competition, but are close enough to draw some people away from the older websites.
Everything gets too big, too popular. It happens. Reddit was at its best 7 to 10 years ago. It's well past its best before date. It has gone mouldy, started to smell, and taste funny. Time to chuck it out.