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.
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?
My brother/sister/mom/friends might use the e-mail once or twice per day, while they would use IM on mobile dozens of times per day.
When you see people on public tranport or on the street writing on their mobile, what do you think they are doing?
Most of the time they are using some kind of IM, not writing an email.
iMessage is the solidified IM platform I see most, as if you have an iPhone, you have iMessage.