Ok. to add to a few points I've seen here.
It is possible to have a D/s relationship online, I have seen many who do have it exclusively online, and for some it is a good way to experiment and see if they will like it, the problem you have to be careful of, is some only want to live out a fantasy, but others live the lifestyle on a more permanent basis, whether this be 24/7 or, like sprite, on occasion but still not limited to the bedroom. There are some who have started out online and found the perfect Dom/me or sub for them, and have met up in real life.
My personal view, is it is preferable to be together in real life, but online has some advantages too, for example you can test out new things and it is easier to stop it if you don't like it, also, if either side is getting uncomfortable with the relationship, whatever the reason, it is easier to get out and away, I'd like to say it doesn't happen, but some do feel the relationship has gone somewhere they are uncomfortable with and want out. This is easy online, but in the real world, and I am very sorry to say this happens, but on occasion one side will not let the other go, this happens with subs, but is most common in Dom/me's, they feel they still own the sub and wont leave them alone, like I said, this can happen with a sub but is less common.
A point Jersey brought up, I too have seen Dom/me's jump on and proclaim they are looking for a sub to collar, while I know this isn't always the case, but often people who say that do not understand the relationship, they are often of the belief that being a Dom/me is easy and means you get to have someone to live out any fantasy you want for you. This is NOT true, and idea's like that are why this lifestyle still brings up bad images to those who don't understand it.
Being collared is a big deal to a sub, I waited a long time before I actually gave my sub a collar from me, if you read my stories you'll see she did wear a collar that she already owned, but what I didn't add in my story to save time, and space was we talked for a very long time about it, what we wanted, our limits and other things before she put that one on, and it wasn't even my collar, that one was hers, I waited for so long to give her mine because I wanted to make sure that we both wanted it.
Those that truly just want a Dom/me straight away to collar them, or when a Dom/me wants to collar a sub straight away are, I am sad to say, often trying to fill a gap, for example if a Dom/me had a sub that left, or a sub had a Dom/me that left, they may rush into another relationship to fill that hole, sometimes this works out, but often not.