It sucks, but it's true. You can argue with the ideological purity of it all you want, but it boils down to the fact that you must choose between purity, and staying in touch. Period.
In my experience there was a phase where that was an issue, but people learned relatively quickly that saying "oh, sorry I didn't invite you, you weren't on Facebook" doesn't go over well, and is really hard to spin as the others fault.
With Whatsapp, they have data (text of conversations) and some metadata (who I'm talking to often) but its much harder to draw inferences from that. I don't see any ads, and my behaviour isn't tracked. With Android M, I'll strip it of all extra permissions like location and microphone, further boosting my privacy.
tldr - Its a lot harder to monetise me on Whatsapp.
Your cellphone constantly broadcasts your location to your carrier and to Facebook and other apps, if you have them installed. If you analyse this data, it is possible to make out where you work, where your home is, where you like to hang out (and with who), and where you take vacations. Further, it is possible to identify a person uniquely from just 4 points of location data if you have a database of everyone's location data. As former NSA general counsel Stewart Baker said, “Metadata absolutely tells you everything about somebody’s life. If you have enough meta data you don’t really need content.”
For more uncomfortable and scary revelations like this, I recommend checking out Data and Goliath by Bruce Schneier.
I just wanted to add another voice saying that this isn't remotely true. I have a large and close-knit circle of friends, who work in technical fields, but Facebook isn't our thing. A few of my friends use Facebook a lot, but many use it rarely, and many don't have an account.