I guess it wouldn't be difficult to implement on something like @ecency, using the memo field to send encrypted messages. Although it does cost a fraction of hive per message to use. I would likely still use it. But real instant messaging should probably be implemented as an off-chain function.
There are several open source messaging standards to pick and choose from.
Maybe the frontend developers could settle with one such standard and develop cross platform instant messaging based on it.
That is kind of what I was thinking. For the encrypted communication at least.
It is true that it would cost 0.001 hive at least, but if we both where chatting, the 0,001 hive would be going back and forth so in the end it would cost almost nothing.
It would increase with the addition of more members to the group chat.