If you want to take advantage of, and get noticed by, the wise crowds, the challenge is to be where the people are.
(That is a message that holds for libraries as well.)
Where the people are, as far as I can tell, is mostly looking at the first page of regular Google search results. But there are some communities. It's hard to tell what the real ones are though. Many sites are much bigger in "buzz size" than they are in actual membership. That being said, there are at least some Usual Suspects:
delicious for bookmarking
Flickr for photos
One of the reasons I think Picasa Web Albums is misguided is that ship has sailed. Just give up and support Flickr already.
For bookmarking, however, delicious doesn't quite do it for me. I like Furl better.
But then, I'm not participating in the delicious community conversation.
So I experimented with copying my Furl bookmarks to delicious, here is the result
Mac OS X comes with Python, so I just ran it at the command line.
Since I have 1561 bookmarks in Furl, with multiple tags, and only 599 made it to delicious, with only one tag each (UPDATE 2006-06-16: And it splits multi-word tags into multiple tags, instead of joining them), I think it is only a moderate success.
A better way to maintain both sets is to get one of those bookmarklets that lets you send your bookmarks to multiple sites at once. Right now I am trying a bookmarklet built with Site Submission MultiTool - Alan's Marklet Maker. It brings up both interfaces, with the URL info filled in - about as good as you could do without a lot of custom work I guess.