This is probably because:
- IMs are already formatted to show the end of each message (chat bubble, etc), so a period communicates extra 'finality' beyond what is necessary;
- Sentence fragments are normal and expected, but seem weird with a period at the end;
- Using formality in an informal setting can create a sense of emotional distance;
- IM cues like these are likely established and spread by people who have mostly used IM in non-professional settings, with friends and significant others, and those norms are then brought with them when interpreting the IMs they receive in a professional setting. If you never communicated with your significant other via text message, you've probably never needed to express as many subtle emotional signals into your texts, and so just treat them like emails. But if you have, then you eventually pick up how powerful punctuation can be at communicating emotion in that medium.
Eg.
> I'll be home late tonight
>> Okay
VS:
> I'll be home late tonight
>> Okay.
> I'll be home late tonight
>> Okay.
The only way in which that strikes me as "weird" at all is that the second person responded in a different style than the person they are responding. That is, there's nothing inherently weird or awkward about the period. Imagine, for example, this instead:> I'll be home late tonight.
>> Okay.
I don't think anybody would bat an eyelash at either part of this exchange, because they "match" in style.
Because while I totally agree with the individual that you are replying too, I TOTALLY see how it could seem ridiculous to someone with the periods-end-sentences perspective.
Let me try and come up with an ideal example. Consider this IM to a friendly teammate concerning an important (but not critical) meeting:
> did we get everything sorted out for the meeting this afternoon? we cant screw it up again
> Did we get everything sorted out for the meeting this afternoon? We can't screw it up again
> Did we get everything sorted out for the meeting this afternoon? We can't screw it up again.
In my world, the first two communicate pretty much the same thing. In the first example, you could probably even replace the question mark with a comma. I would likely send the second message, as I prefer descriptive, detail-adding punctuation. I would be less likely to send the third message to a young coworker because it seems standoffish.
Having said all that, any of the three messages would suffice. Someone mentioned elsewhere in this thread, but we - or at least the younger generation - have all become quite good at code switching. Crazy times
Hey.
Or this exchange: Statement: We got the tickets for tonight!
Response: Yay.
It comes off as very flat or dry. Unenthused. A plain "Yay" with no punctuation would be better here. I wonder if this is the context that we're missing and others aren't providing, because I almost always use punctuation, including periods, in my messages. No one has ever complained. But with short messages, like these, I would never use a period. For full sentences, however, I can't think of any occasion outside being rushed where I wouldn't end it with a period.