The ellipsis, like much punctuation, has several uses. It's a fine example of the flexibility of the English language.
As has been said, it's most often used to signify a 'trailing off' into thought or contemplation. This is called Aposiopesis.
It's also used as a sort of 'pause' or short silence mid-sentence.
I use the ellipsis for both of these uses, but I won't use more than one instance of it in a single sentence unless my character's suddenly awkward, hurt or hesitant. Since use of the ellipsis suggests an inability or unwillingness to respond, it should be used sparingly for characters who don't have communication problems.
As Graeham said, a simple " ... " as reply doesn't really give others much to go on. It also leaves things unclear as to whether you're going to post again, or if it's their turn. In that case you'd likely be better off emoting a silent response.
The "..." as response does have its uses, though. I find it good for conveying that Bryn either can't believe the stupidity of something that was just said to her, or that she's about to turn nasty. It's the 'calm before the storm', if you will.