It seems that some sites disappear into a black hole (such as Kuro5hin) and some become dwarf stars (like Slashdot) - they're still burning, just not giving off much heat or light.
As a long-time member, I'll answer your question directly and honestly: people looking for a well-moderated discussion community that don't tolerate right-wing attitudes very well. Metafilter is, in a very real way, one of the last bastions of the old web.
But well, MeFi is more of a community than a company and as far as being a community goes it's done about as well as anything on the web.
Matt has been great at keeping a community going.
Kuro5hin, Slashdot and many others have come and gone but MeFi keeps on going.