Is it possible that this other organization's use of "your" hashtag can be a GOOD thing? Or could be leveraged as one?

I'm actually struggling to think of a reason why a Twitter user would WANT to be possessive of a particular hashtag, though. The main benefit of using hashtags is to offer the community a way to aggregate tweets by topic, no matter which user is doing the tweeting. Furthermore, using hashtags is a great way to trickle into the other users' search feeds, particularly if the tag's purpose is clear and used by many. Unless you're a giant org that tweets prolifically, has a massive following, AND wants to organize tweets by topic for the benefit of its followers, I can't think of why another organization's co-opting of a hashtag it finds useful would be a bad thing.

I know I'm missing some critical details about what the situation is (like what the hashtag is and how it was/is now being used), but try to see if this can work to the benefit of both your organizations by agreeing on how it's to be used.

If I'm way off-base here, however, the bottom-line is that you can't rely on using a specific hashtag that you hope no one else will. Your options are either embrace the idea that others will use it for similar purposes (if they're not, though, have a chat to see if you can come to a friendly agreement on its use for mutual benefit) which you can use to your advantage by gaining more exposure for your org or cause, or come up with a new strategy to replace the one that required the "possession" of a particular hashtag.

I have concerns about this. Does anyone "own" a hashtag? Isn't the point to have a way for people with common interests to post information that is interesting?

I don't think anyone can stake out a hashtag. If Tweeps want to post information that would be of interest to a community that is using a hashtag, that's a good thing. That's the entire point...to allow people to search for information that would be of interest to them.

I frequently post information that is interesting to the community that follows another organization's hastag. I don't see that as a problem, because it is valuable to that community's members.

I have also experienced a group that started using a well established hashtag that didn't "belong" to anyone...it was just for a large community of people with the same interests. The other group was a temporary group following an event. They came and left.

I don't think you should call the other organization unless the content is offensive or they are trying to defame or otherwise undermine your organization. If they are just posting content, you have to let it go. Social media comes with some element of loss of control. That's the deal.

In response to Matt: I understand where you are coming from.  For example, the hashtag #healthreform is pretty general and you might want a lot of people to use this hashtag to easily search for folks talking about topics related to Health Reform.  However, that is a topical hashtag.  I work with Federally Qualified Health Centers and we use the hashtag #fqhc.  Using a hashtag that is an abbreviation (or letters that stand for something) is when hashtags become tricky.  While FQHC to me stands for Federally Qualified Health Centers, what if another organization out there that is unrelated to health also uses the same letters, but to stand for some other kind of organization or topic.  So, for example, if a bank started using our hashtag, it would become pretty confusing when folks searched for the #fqhc tag and got a mix of health and banking related tweets.

I just wanted to stress the point about using  it often, because if you don't any search results in Twitter disappear after about 2-3 weeks (from best guess) and hashtags.org only archives for a month. I've heard about wthashtag.com but don't know how far they go back.

The important thing about a hashtag though is to research it before you start using it. If it's already in use, are those tweets related to  your org or event? Does it make sense to use the tag? Is the tag short enough so that you aren't taking away from valuable character space?

Hashtags are a great tool to help create community, you just have to be smart about how & when you use them.

I would say if an organization wants to aggregate their tweets and is worried about using a hashtag because others will/can also use it, is to setup something that uses Twitter Search that way they can control what someone see's by whomever it is originally sent by, example (@eeus).  "With Sender" being the criteria.

A hashtag I think is the wrong technology for an organzation if this is what they're trying to accomplish.  If they really need a hashtag, I'd recommend picking something no one else would ever use in a million years, #qqzzyytt  although usually you want them to make sense.

Very interesting post :-)

I appreciate this post. Hashtags can be used for many reasons - some of them more organic. To follow up on the previous comments, I would not consider it an issue of ownership but it can be an issue of reputation and meaningfulness. Be aware that if you post in a tag that is already in use you run the risk of confusing those who are participating. I like Vicky's idea of using something completely obscure... but remember, one person's obscurity is another person's meaning. One useful place to check would be http://hashtags.org.