Brb = Be right back
... = Silence
So the person who said brb... either
A) Wants you to disapear
B) Has something going on yet they dont know when they will be back
C) Something really odd is going on like if cyborg pirate ninja jesus was trying to sell them cookies
D) Adds on the ... without knowing what the damn thing means.
Billybob: OMFG lost soooo much money at the dog races today.
Jimbobjoe: Well maybe you should bet on the rabbit next time! lulz
Brandi: Hey Steve whats up!
Steve: n2m u?
Brandi: Nada brb...