And they'll know who you contacted and when. That allows for a lot of things to be guessed. Especially with hundreds of thousands of messages per user available to them.
Yeah, but my ISP has to follow the local laws and I can easily switch ISPs if I dislike my current one. Try that with Facebook.
Yeah, I might use a VPN but I'd also have to force all my contacts to use one as well.
Your ISP knows the same when using a web site via HTTPS. They can see you send a request to the website but not what it contained.
You could also use a VPn or TOR if you want to hide your IP.