Activity stream chat?
For some reason, people are still able to talk with me on my stream, even though the Activity stream has been removed. I get notifications that people have posted on my stream, but I can't see the full message. Also, when I go onto other user's streams, I hover my mouse over the spot where the chat option used to be, but it isn't there anymore.
How can people talk to me on my stream, and how can I do the same for them?
They do not have access to your stream or their own. Everyone lost their streams.
The post box is still available when you look at a Duo user's profile and some are still using that box to post messages to streams that are not there, but the receiver of the message still gets the notifications from the bell and (if applicable) in e-mail as well.
From the other user's perspective:
1) If on the old site you can maybe still post. But if you are not in his friend list (assumed) and or you do not have your email notifications on then you will only see an alert but can not read the notification message itself, it should arrive via email.
2) If on the new site (=the 'Activities' link does not show at all anymore in the menu) then nothing is possible at all anymore.
This behavior might maybe vary temporarily and or might be dependent on your user account also.
The general expectation is that all this functionality will be removed soon.