1. Turn on CSS Classes
Go to Appearance > Menus - and first click on Screen Options at the top. Ensure CSS Classes is ticked.
2. Facebook
Add a Custom Link. Replace # with the link to your Facebook page. Add this to the CSS Classes field: fa fa-facebook
3. LinkedIn
Add a Custom Link. Replace # with the link to your LinkedIn page. Add this to the CSS Classes field: fa fa-linkedin
4. Search
Add a Custom Link. Add this to the CSS Classes field: fa fa-search dark