I usaully don't worry about it. It's not like I'm hanging up on someone, it's a message board, not chat.
There are many reasons...... I may get a phone call, someone comes to see me, I gotta go eat, have to take a dump, I get sleepy quick, there's a show on I wanna see, the weather makes me loose wireless access, I have to go do something unexpectedly.....The list can go on.
If it's just me & one other person, I might 'sign off' and let them know, but that usually means a PM.