There's a subset of my Facebook friends, mostly older/family who are slow to reply to messages on Facebook and only use Facebook in 'down time' or whatever but are available by text all day. Since they got smartphones years ago, by text has meant by Whatsapp.
The way I was looking at it was people will get messages on WhatsApp but won't reply immediately. Same on Facebook. But people will also visit Facebook.com when ignoring messages so you still get online status info even if they aren't specifically using the chat. If you could look at WhatsApp and Facebook and combine the data you'd probably get a really accurate overview.
