Great question Jasleen. Interestingly, employee advocacy and social employee advocacy are interchangeably used terms. At the same time, employee advocacy has existed since long including before digital media. Building a strong culture for customer excellence and sensitising every employee towards customers lays a strong foundation for employee advocacy.
Zappos is a great example of it and was at the core of Zappos success. Have a look at https://www.zapposinsights.com/culture-book. HCL's employee 1st and customer 2nd is an interesting example of employee advocacy in Indian context. Other than social media, emailers can be an effective medium to build employee advocacy.
Does this help?