If you look at
http://delicious.com/tag/berlin+hotel
you will see that a particular place was bookmarked 4 times on one day
by 4 different users "who" have only one bookmark each, that one.

This is clearly advertising.

Is this explicitly allowed, or not allowed? If this is not allowed, do
you make any effort to exclude this sort of practice?...

I'm just curious. I have nothing against the Hotel 4 Youth.

Thanks,
Matthew