I've had this enabled for 6 months and I've seen websites stopped working as they should, e.g.:
- I couldn't reset my password in Twitch, it required (for some reason) access to both Facebook and Google servers. I had to temporarily disable the `/etc/hosts` restrictions
- National Geographic site fetches jQuery from Google and the website is unusable
- No more embedded Google Maps on many small websites (like local restaurants etc)
- Say bye to embedded YouTube videos
- Many small websites just display in blank
- Other websites have default ugly fonts but are still usable
So Google and Facebook's reach is not just related to direct visits to their sites. Essentially, without them, it's a very crippled internet. The one site that works great is Wikipedia, even its videos are self hosted.