On average, each tweet is read by about one person, and an average whatsapp message is read by one than more person.
There are tweeter messages that are read by millions, but they are offset by many tweeter messages read by no one. The average no. of followers of a tweeter account is 1, and 5-10% of all tweeter accounts are bots.
The distribution is different, but there are a lot of similarities that warrant comparison.
Twitter isn't really comparable to WhatsApp. How many people read a tweet compared to a WhatsApp message.