A habit I've gotten into since using fb messages, Viber/WhatsApp/Telegram is sending lots of messages with less words. So instead of one long text I might send 4-5 short ones. That will get you to 50 messages easy if you're texting a lot of friends.
I guess group messages would make the most sense with this stat.
At first i thought it sounds a little high, but then i started thinking about my average communication protocol(IRC). I send a lot more than 50 messages a day over IRC, but i may not be average.
That's around 4 per hour. Doesn't seem unlikely to me, especially when you consider the general melding of texts and IM.