I haven't made use of Delicious for a long time now... logging me out isn't going to get me back either!
Wanna bet?
Every time someone clears their cookies they have to log back in. If someone is such an apathetic user of the site that they can't even log in, they'll stop using the site the next time they clear their cookies anyhow. All Delicious is doing, in effect, is clearing everyone's cookies at once. I'm sure there will be a slight dropoff, but this article exaggerates.
The point was that a bunch of their "users" don't actually use the site, they just never bothered to logout