Do they allow synchronous (immediate) or asynchronous (there’s a lag between a comment and reply) communicatio?

[Jelly]

For blogs, wikis, chat rooms, forums and also Social Networking Sites (like Facebook)

[Ephemerist]

For the ones that you named, no, because those are not peer to peer services. IRC is pretty close but that passes through a server too.

I haven't seen Powwow in many years but that used to be a 'type a key see a character' kind of chat with up to 8 users on a screen. It had VoIP, whiteboard, and was able to give control of several browsers to one controller for group surfing.