With a close friend here I'd just be surprised but I wouldn't find it rude.
Perhaps there were plenty of reasons why they had to leave and saying goodbye was not in their mind. Could be tons of reasons.

Not necessarily.

Could be accident or illness - no way of knowing.

It makes them inconsiderate...

I think so.

Yes

It happens..

It can be very hurtful, but everybody does what they do for a reason. We can't always know the whys. That's why it's important to say the things that need to be said while they can be said... with each person, on or offline.

Eh.... I think you gotta expect that and not get hurt feelings over it...

I believe that some people find it easier to just go.

Not always.

It is quite rude. However, it's just one of those things, nothing you can do.

If that's what you would usually do then maybe so but I wouldn't be offended if someone did that to me. People log on and off as they like, I find.

It is but it does happen. Once I thought a person passed away, only to find out he was star struck in a new relationship.

If you're in the middle of a conversation it is. 😕 I always say goodbye when I'm leaving...👋🏼

Not at all my real friends did the same thing.

No, they don’t owe an explanation

Yep an also selfish