IM is honestly the number 1 social media right now. Its real communication with people you actually care about and not a popularity contest or marketing and no algorithm trying to tell you what you should be looking at.
I guess you can. Email seems to be more popular, though. For any given person, there's a higher likelihood they have email than any particular IM client.
In many countries in Europe, WhatsApp is just about ubiquitous, and used by the average guy way more often than email.
The same (from what I understand) for WeChat in China.
Yes, if you need to communicate outside of your local circle of family and friends, e-mail is still king (for business communication for example, no contest), but how often does the average guy does it?
I agree that more people have an email address than a particular IM client, however, a lot of people outside of the tech bubble don't use email much anymore for personal communication.
iMessage is the solidified IM platform I see most, as if you have an iPhone, you have iMessage.
In Australia SMS is still widely used because of IM fragmentation. A proprietary IM platform will never take over and we will stay in the current mess until one of the open source standards become well used.