E-mail analogy is better. Imagine a service, which only allows using one e-mail client and doesn't allow sending e-mails to anyone outside itself. I wouldn't want to use such service.
Interestingly, such a system (e-mail like system which only supports first-party clients and can only send to and receive from other users of the same service) is a common feature of web discussion fora and social networking sites.
You might not want it, but such services haven't exactly failed in the market.
Your example isn't good, since forum messaging has limited scope, while e-mail is generic. Instant messaging can have limited scope in other scenarios (let's say you chatting with some support representative for some service). It can be generic as well. Here we are discussing generic instant messaging. Therefore it's proper to compare it to generic e-mail. When it's artificially limited by the service walls, it's unacceptable.
such services haven't exactly failed in the market.
Indeed, services like AIM, Skype, Whatsapp and etc. have many users, and are presented as generic instant messaging services, yet they don't allow their users to communicate with others. It's somewhat surprising to me, that people are ready to accept that fact. For some reason they'd see cutting their e-mail off from other servers as unacceptable, but the fact that they are confined within the network of proprietary IM clients doesn't bother them, even though because of that they need to use many clients or protocols to communicate with different networks having an account on each.
> Froum messaging has limited scope. E-mail is generic.
Making a system closed by boundaries automatically gives it limited scope, but forum messaging isn't any more limited in scope than results from those closed boundaries. Ditto with social network-based closed mail-like messaging.
At this point, I'm not even sure what your point is, since you said my examples were bad, and now seem to be saying that your initial criticism doesn't apply to one of my two examples, and that you wouldn't compare the two examples I gave with each other.
My point is, that there are scenarios when closed messaging networks are acceptable, and there are scenarios when they aren't, but despite of that, people tolerate them in case of IM, which allows various messaging services to get away with it. I said that I really don't understand why that happens, since in case of e-mail they don't tolerate it.
Hangouts, FB messaging and etc. are generic cases, not the ones which should be acceptable as confined environments (like your example of the forum messaging).
