Just like all things tech, instant messaging has evolved a lot since the early days of the internet. This is to quickly summarize what has gone down in the past decade or so in terms of messaging one another. Spoiler: login-required protocols are out, login-free protocols are the way to go.
This is probably the most ancient IM. Each user account is assigned a "number" according to their initial registration and users are required to log in every time. The idea of alphanumerical username was too advanced.
Email service providers entered the IM market with MSN quickly taking over as the most predominant IM service globally. (With the exception of the US which preferred AIM slightly more.) Each user account is represented by an alphanumerical username which is just your @hotmail address.
Facebook made a breakthrough in the IM world with their integrated messaging service. It started with Facebook Message, and then Chat was introduced. Eventually, these two are combined as one and became Facebook Messenger as we know it today. This is still login-required but with most people permanently logged into their Facebook account on their desktop and mobile, the actual action of logging in became obsolete.
A not-so-popular service which has the benefit of being integrated into Android phones and GMail. Once again, it's still login-required but most people are permanently logged in anyways.
More a VoIP service but there's an IM aspect to it. I wouldn't say it's very popular but some people depend on it. Login is required.
And finally in comes the truly login-free IM service. This type of IM links to a cellphone number and no login/password is required. The obvious requisite is for the messaging party to know each other's phone number.
I would think that most people use FB and Whatsapp the most today with GTalk here and there. Let's revisit this post in say 3-5 years time and I'm sure things will be very, very different.