As the above chart (source) illustrates, email click-rate has been steadily declining.
Whilst open rates seem to be increasing - largely driven by mobile - the actual engagement from email is nosediving.
They acquired over 10 million new users within the first two weeks of launch and things were looking positive.
Don't forget that you can also tap into platforms outside of Messenger as well, for example, Slack, Telegram and even SMS.
Whilst the payout wasn't huge within the early days of Amazon, those who got in early are now seeing huge rewards, with 38% of shoppers starting their buying journey within Amazon (source), making it the number one retail search engine.
Some studies are suggesting that Amazon is responsible for 80% of e-commerce growth for publicly traded web retailers (source).
I don't think many people will argue that, whilst this is the case now, things are only going to get worse.
As with many 'organic' channels, the relative reach of your audience tends to decline over time due to a variety of factors.
Search for Chatbot:
In email's case, it can be the over-exposure to marketing emails and moves from email providers to filter out promotional content; with other channels it can be the platform itself.