I'm probably talking total rubbish here.
Any community tends to go downhill as it expands. It's just a matter of statistics as to when it becomes a sludge pond. After all, initially it's composed of the founders and a few like-minded enthusiasts. These may well have left an already existing community to found a new one, focused tightly on some specific subject. Or they just want to escape the decay of the old community.
The next stage is the golden age: enough people so there's always something happening, not so many that there's more than the occasional lowlife (there aren't necessarily many, they're just noticeable).
After that, the floodgates open. The community attracts vast hordes of users. No matter how enlightened the policies of the staff, they can't cope with as many people as the servers can. Although the percentage of tards/lowlifes doesn't change, their actual number increases.
There is a bright spot: as the community expands, the probability of it fissioning into new sub-communities also increases. This same process can apply in miniature for those sub-communities. This does depend, however, on the parent community not having decayed into sludge. This happens when the number of tards reaches a point where it totally overwhelms the ability of the staff and sane users to block them out. At this point, the community reaches hemorrhage status: real users leave in droves, perhaps decamping to newer sites, leaving the old place to the tards.
The community lifecycle:
1) Founding, original enthusiasts.
2) Growth, golden age of intelligent, well-meaning users.
3) Wider popularity, tard invasion, site begins to slip into social networking/hangout spot. Multiple sub-communities in evidence.
4) Population critical mass. Staff no longer able to keep order/focus.
5) Quality decay. Users start leaving in greater and greater quantities.
6) Sludge lake. Tards overrun site. Sturgeon's law rules with iron fist.
7) Population decay. Even tards leave. Last one out closes door.
8) Death. Dusty corpse lies around until something expires or is terminated.
9) Vanishment. The Wayback Machine is the only hint of it's existence.
Of course, this doesn't fit everything. In particular, if the original focus is a game, or a series no longer putting out new content, then it might go straight from 2 to 7. In stage 8, what generally expires is hosting or DNS registration. Termination involves the owners removing parts of the site outright.