I think a productive response is to direct people to things that will let them sate their lust for online multiplayer Dwarf Fortress so they can get over it, so I'll list what I can think of as things that scratch the DF itch and have some form of multiplayer.
Wurm Online is the thing that occurs to me as the most successful DF-like multiplayer game. It's nowhere near like Dwarf Fortress, mind you. It's not, in my opinion, very fun. Some people really enjoy it, but it takes a long time to get things done. What makes it DF-like is you can mine out ground, make caves, build buildings, craft things, etcetera. These things will take a very long time to do and are very obscure to learn. Some people run cities and such, I think you can make wagons and have mounts, so if you have enough friends or others to cooperate with to provide a vast pool of free time to sink into realizing your dreams, yeah, you could pretty much play it like multiplayer DF.
'Love' is an upcoming (cross your fingers) online game that will have many sharded worlds that handfuls of players will share, meaning that it can allow individuals to rapidly change the environment (since they aren't ruining the fun of hundreds of other players if they stay up all night and terraform the world). Kind of hard to google 'Love' and find this game so I'll just post the link for those curious.
http://www.quelsolaar.com/love/screen_shots.htmlAlso there was a Dwarf Fortress clone some guy made on BYOND. It was kind of interesting but the server is not freely available, server availability and stability can be very iffy, and it's nowhere near as fully featured as DF. There are no NPC caravans, there's no automation whatsoever, so it can be very monotonous and wrist-injuring to run a hamlet let alone an empire. I did find it entertaining for a few days, and it's definitely a great way to learn that a limited version of DF, multiplayer, isn't /that/ great of an idea.
http://www.byond.com/games/Ginseng/DungeonMasterFor something completely different, and quite awesome in its own right, there's Cortex Command. It DOES NOT have online multiplayer! It has split-screen (or multi-display?) local multiplayer, so if you got buds and some USB X-Box controllers you're all set for crazy fun. It's a 2D sci-fi action game, but you can build bases above or below ground (before play begins), mine gold, and defend yourself from hordes of AI attackers. There's an extensive modding scene. It is in development, and you may experience crashes especially with certain mods. The campaign isn't finished or fully realized. Still, fun.
http://www.datarealms.com/On the highly graphic'd, 3D front, I've heard good things about Red Faction: Guerilla, it has destructible terrain and buildings and I think that's the case in multiplayer as well. However, there is no mode where you build a fortress or a base or anything, AFAIK, and the PC version still isn't out yet. I don't know if the PC version will be modable, there is a worsening trend where developers are making FPSes impossible to mod (Far Cry 2 springs to mind).
I believe there's some kind of Fort Defense: Source or something mod where you can build bases, but I never tried it. What I have tried, and enjoyed, was Source: Empires. Working with a team, the commander can plot out bases, defenses, and research up a tech tree, allowing other members of the team to customize and build ever more powerful vehicles. If the other team blows up your commander's APC, you lose. You kill theirs, you win. Personally I'd say it's fun either way so long as the match has some good fights and such. -- Caveat about its DF-alikeness though, there is no terrain deformation, and a round takes anywhere from ten minutes to an hour to complete so no matter what you build it's like a sandcastle on the beach. Strictly temporary.
edit: You know, I wonder if we could get a sticky somewhere that explains Dwarf Fortress is not planned to be multiplayer ever, that it would be very hard to make that multiplayer work right or be fun, and that there are all these alternatives out there (like succession games) to scratch that itch. It could have a title like, "
Before you suggest multiplayer Dwarf Fortress, Read This First!", and might reduce the frequency of threads like this that pop up.