Well this weekend sees us reaching the 18th session of the Lost City of Barakus camnaign, which is the longest running 3e campaign I've played in, though the first 3.0 campaign that I restarted D&D with in 2003 is only on hold rather than definitely finished for good. Warning, this post will contain a lot of spoilers for the LCoB, so I would not advise players expecting to be adventuring in it to read this. Campaign website here.
So far there have only been two survivors all the way through, both as characters and players. My cleric Cambyses of the Holy Flame and Ravi's Loren Silverleaf, an elven wizard. So far we've seen another four players and their characters leave and had another three join with the latest only coming in this weekend. The other long running PCs have been Kiru, the fighter/ranger; Khai Ningnoon, monk; Armando Gallant, fighter/rogue and Nip Mylan, rogue.
Generally the Rogue and Fighter characters have been shortest lived with the toll of NPC followers/associated killed now totalling seven, due to use of fate points no PC has been killed, but several have been left for dead, with Cambyses having has this happen to him three times, most recently on encountering a Blue Slaad. Generally I think that fate points do help and am houseruling them into my D&D campaign, as I prefer to run at lower levels and with low magic and treasure so less healing is available and this makes it possible for a character to survive the occasional -10 hit point experience.
The ability to keep the campaign going has been down to a number of things, such as being able to cope with occasional player absences on some weekends, using what is more likely to be a free day with Sundays, only having a couple of sessions per month, but always having the sessions. I've had other campaigns fall down with cancelled sessions being a common cause as the enthusiasm gets lost and the players lose track of the game. That's always hard to manage when as with the last campaign I gm'd half the players lived a long way away (2 hours + journey time). I've not yet had enough experience of play by post or play by email games to see how those pan out over the long term.