One of the keys to succesful collaboration is keeping a running summary of the problem-solving process: what ideas have been identified as promising, which of those have been discussed, what conclusions were reached, and so on.

This is hard to do in the stream of a chat, and if you try to do it there, it will be swept off the screen after a few more comments have followed it.

Text boxes in the shared whiteboard are ideal for keeping track of what's happened:

A: There are three possibilities...
A: the angle is 90 degrees...
A: or it's greater than 90...
A: or it's less
B: obtuse, right, or acute
A: yeah, and we can consider those cases separately
C: let's list those cases on the whiteboard...
C: so we know which ones we've tried

But sometimes a quick summary in the chat stream can help the group refocus, e.g.,

Q: let me see if I can summarize what we know...
Q: x can't be negative...
Q: and y has to be a perfect square...
R: yeah
Q: but we still don't know if y has only one possible value...
Q: right?
S: we also know that x can't be bigger than y
Q: oh yeah, I forgot that. 
R: should we collect this on the whiteboard?