This is exactly why Whatsapp got so popular. It's just MMS with few improvements (better image/video/sound quality) and at no price (well, 0.99€/year now, but it used to be free for most of its life).
It even has the same very low barrier to entry - You don't need to explicitly add contacts, it takes everything directly from your address book, which is likely already full of your friends/people you know. Hell, you don't even need to create a new account, it validate automatically your phone number with an sms. While this is awful privacy-wise, and kill the possibility of an easy-to-setup desktop client, it makes everything really easy for everyone, especially those that use their smarthphones for most of their online tasks (which seems to be a big group growing every year).
It even has the same very low barrier to entry - You don't need to explicitly add contacts, it takes everything directly from your address book, which is likely already full of your friends/people you know. Hell, you don't even need to create a new account, it validate automatically your phone number with an sms. While this is awful privacy-wise, and kill the possibility of an easy-to-setup desktop client, it makes everything really easy for everyone, especially those that use their smarthphones for most of their online tasks (which seems to be a big group growing every year).
Nothing more, nothing less.