Back in January, code showed up suggesting a closer sharing of data between the two companies, and now it's arrived whether you wanted it or not. Privacy advocates will clearly be concerned about the sharing of data between two of the worlds' most popular social services, and the two largest messengers.
WhatsApp, on the other hand, says it needs to share data to test out new features in the next couple of months, like "ways for you to communicate with businesses that matter to you" and "hearing from your bank about a potentially fraudulent transaction, or getting notified by an airline about a delayed flight."
It's worth noting the details included in the agreement: your phone number, profile name and photo, online status and status message and last seen status. It doesn't include the content of your messages.
If it all makes you a bit uneasy, try not to worry, it's just the biggest social company in the world working out better ways to target its ads -- and there is a way to opt out, but it could be easier.
Update: This post has been updated to clarify that Whatsapp does not actually have access to read receipts.