I suspect because people don't always know what other communities are around, sometimes contributions are made that may be better suited elsewhere - but you can't post in a community if you don't know it exists.
Accordingly, I've listed below a number of communities that may be of interest to members: I don't doubt there are many others, and welcome further suggestions. These are the communities I have on my Friends page.
Hopefully this may help limit the number of not-really-decay pictures posted here, and also encourage people to post more photos than they otherwise feel they can.
I should point out that there can be much overlap between various communities, and I'm not suggesting any given photo has only one niche into which it fits, but rather that this is an opportunity to broaden your horizons while keeping urban_decay true to the concept for which it was founded.
For the record, also, a few of these are my communities - but that said, some of the others I refuse to post in, because I don't enjoy the pervading atmosphere. Hopefully it balances out.
This post will be added to the community's "Memories" page.
rural_ruin - the country side of the "decay" coin.
deadmachinery - defunct machinery (and artcars, potentially less-dead machinery)
futurecity - "Scenes from great cities"
deathly_decayed - one of many communities devoted to cemeteries. I chose this one specifically because it is exclusively devoted to photographs; but I suggest you "shop around" to find one that suits you.
signs_n_wonders - Vintage signs in various states of repair
look_up and look_down - Exactly what they say.
color_theory - Despite the community name that has (and will doubtless continue) to create friction, the concept of this community is much broader. The moderator has stated that "This is not strictly a community about photography. it is a community about color. ... People take snapshots of objects to talk about and show - like show and tell in kindergarten. the photo itself is not the object being brought to show and tell."
texture - Self explanatory - but don't include text or your post will be deleted.
colorandpattern - Colour, pattern and texture.
secret_gardens - "Windchimes, birdfeeders, statues, ponds, fountains, ivy, moss, waterfalls, mushrooms, birdbaths and particularly interesting plants and trees!" Regrettably rather quiet - perhaps there's a similar community out there I haven't found?
vintagephoto - Old photographs; and foundphotos - Found photographs
found_objects and mundungus - neither exclusively photographic communities, but they do provide a place for the stuff that doesn't fit anywhere else.
The founder of this community encouraged links to websites devoted to urban decay as part of the initial concept. People are naturally welcome to continue to alert members to such sites: the only thing I ask is that you don't re-post images without permission, most particularly if you steal their bandwidth to present them here; and even more pertinently, please do not take other people's photos and try to pass them off as your own. I don't want to have to go through that again...