In order to synchronize Insights User Segments, or synchronize User Fields (attributes or metrics) to Salesforce you will need to configure how to match Insights User records with Salesforce Contact records.
Ensure you have completed prerequisites:
Your Insights tenant is set up to ingesting data from Salesforce
Open the Profile and Settings menu in the upper right corner of the page.
Navigate to Settings. The Settings page opens and displays each administrative setting on the page.
Click on Manage Salesforce Integration. A listing of Salesforce configuration options will be presented.
Click User Matching. You are presented with a form that lets you map User unique identifier fields between Insights and Salesforce.
Now establish the mapping between Users in Insights and Contacts in Salesforce by selecting the Insights ID Attribute field in Insights that uniquely matches with the Salesforce ID Field on the Salesforce Object you select on this page and click Save. Note that Insights lets you map Users to any custom object in Salesforce that you may be using to store your users -- you are not required to store your users as a Contact in Salesforce, although that is the most popular configuration among Insights customers.