How do police arrest Facebook users?

[Bob B]

Depending on the nature of the post:

Posts on the internet are (or can be) logged to record the IP address that's used to make the posting. That IP address identifies the person's internet service provider. The ISP also keeps logs of which of their accounts was assigned that specific IP address at a particular time.

From the posting IP address, the ISP gets subpoenaed to identify the account that made the posting, While you can't be personally identified as having been the person who made the post, your ISP account can be found by the police. The account information at your ISP would give them your home address and phone number.