'Follow' Icon Not Available on Records or List Views

To enable 'Chatter Feed Tracking' in Classic UI

1. From Setup, click Customize | Chatter | Feed Tracking
2. Select an object (for example, Leads, Contact, or Accounts)
3. Select Enable Feed Tracking
4. Click Save
5. Navigate to a list view or record page for the enabled object. The 'Follow' icon should now be present.

To enable 'Chatter Feed Tracking' in Lightning

1. From Setup, in the 'Quick Find' field search for Feed Tracking
2. Select Feed Tracking
3. Select an object (for example, Leads, Contact, or Accounts)
4. Select Enable Feed Tracking
5. Click Save
6. Navigate to the record page for the enabled object. The 'Follow' icon should now be present.

Note: If the page layout is not feed-based type, the 'Follow' button is set to 'Hidden'. You can unhide 'Follow' button via Lightning App Builder by following these steps:
 

1. While in an Account record, click Edit Page in Setup
2. Select Highlights Panel
3. In the detail section on the right side, deselect Hide Follow/ Unfollow button (desktop only)
4. Save.

Lightning Experience: Follow Button on List Views Not Supported

Note: The 'Follow' button is available only on the record page in Lightning Experience. If you'd like Salesforce to add the 'Follow' button to list views in Lightning Experience, consider voting on this post in the IdeaExchange.