According to the latest update Whatsapp claims it fixed an issue that was showing the user's phone number in Google search results. The security researcher Athul Jayaram reported that phone numbers of Whatsapp users who used the Click to Chat feature were being indexed in Google Search.
So if you don't know about Click to Chat feature, It allows you to create a link with their phone number in plain text. According to Jayaram, because the links don’t have a robot.txt file in the server root, they cannot stop Google or other search engine bots from crawling and indexing the links. Jayaram says as many as 300,000 phone numbers may have appeared in Google search results, and they could be found by searching “site:wa.me.”
After getting this update Facebook replied, the bug was already fixed and the phone numbers found by Jayaram were old results cached by the search engine.