You both can and can't control what your users decide to do with your application. After the community starts developing, though, your users can and most certainly do decide what YOU do with your application.
Hypothetically suppose, for the sake of argument, that the active Loopt community is currently gay men seeking companionship. If that is true, then Loopt is essentially doomed to use for any other purpose unless they excommunicate their current community and start over. If they try to rebrand themselves as, I don't know, Loopt: THE Place To Be For Tech-Savvy Entrepreneurs, then someone joining the non-existent "new" community will get contacted by the "old" community and, well, that's all she wrote for that user. Repeat for every user in the "new" community.
Similar examples of capture-by-community : Orkut can't be a global social network. Youtube can't have intelligent comments. Digg can't ever appeal to an older, femaler audience. Star Wars Galaxy can't be WoW, though they wanted to be (and promptly excommunicated both their old and new communities). etc, etc
Then the solution is not to rebrand to lose the existing audience, but to rebrand to keep that audience, and re-use the software in a new product with a new audience.
Most of the "cants" you referenced were likely not really possible, given the sort of product that they created. Social networking is most interesting with proximity. User-uploaded videos are mostly stupid, and attract stupid comments. Digg is intimidating in its design, which appeals to young male geeks more than old female nongeeks. People who want what WoW offers are already spending all their time playing WoW, and the same game with different graphics won't pull them away from the characters and relationships they're invested in. etc, etc
It's not necessarily a bad thing for users to have an effect on the direction your product goes. In fact, if properly harnessed, it can be incredibly positive.
Nah. Disco became mainstream, yet launched from gay clubs, which are in fact hookup-applications. There are quite a few other examples. Apparently these things do cross over.
"Have faith, Loopt, if your idea goes big somebody else will be around to capitalize on it!" does not strike me as something which would warm the heart of an investor.
Disco is a different beast. It's the synths that are mainstream not disco the genre. Synths were used by many other bands as well so I'm not sure what you mean by launched from gay clubs?
Same with LiveJournal and fandom, I guess. And they tried to get rid of fandom to reposition themselves towards a more mainstream audience, but that just ended up pissing off most of fandom and their new audience.
Hypothetically suppose, for the sake of argument, that the active Loopt community is currently gay men seeking companionship. If that is true, then Loopt is essentially doomed to use for any other purpose unless they excommunicate their current community and start over. If they try to rebrand themselves as, I don't know, Loopt: THE Place To Be For Tech-Savvy Entrepreneurs, then someone joining the non-existent "new" community will get contacted by the "old" community and, well, that's all she wrote for that user. Repeat for every user in the "new" community.
Similar examples of capture-by-community : Orkut can't be a global social network. Youtube can't have intelligent comments. Digg can't ever appeal to an older, femaler audience. Star Wars Galaxy can't be WoW, though they wanted to be (and promptly excommunicated both their old and new communities). etc, etc