Right now they are floating on viral momentum. They are a few stupid decisions and one neat alternative away from going the way of Digg.
I do agree on the neat alternative piece, but all of the ones that have come up in the past had some sort of flaw (or in some cases deliberate slower growth) that caused them not to go viral (Voat, Tildes, etc)
The blue line (reddit) does indeed keep going up. But it's barely noticable compared to the red (facebook).
I suspect in general the entire "thing" that is Reddit is on its way out - just like how craigslist killed classified ads and itself may be slowly dying, Reddit killed forums and now is slowly dying, there may be no replacement.
Def not perfect and long way to go but so far it's been fun!