I actually assume people are ignoring me unless they say something.
Maybe that's because I do apply a sense of etiquette online as I do off but it's SO easy just to tell someone "Hey, I'm a tad bogged down at the moment and may be slow to respond."
And saying "heya" is not even close to shouting in a busy room O_O
Text will pop up, windows will blink at you, and sometimes even make a loud and obnoxious noise.
I have never understood why people "don't notice" all that.