Seema: Social Media Marketing is one of the prominent avenues in Digital Marketing. Given its importance for almost every industry, there's a demand for skilled talent in Social Media Marketing. At the same time, the rate of growth of Social Media Marketing has slowed down over last 1-2 years for following reasons:
It's no more a buzzword; the novelty factor associated with Social Media Marketing is over
Compared to other digital marketing avenues such as SEM & SEO, the direct contribution of Social Media Marketing in growing an organizational revenue is not so significant
The value associated with managing presence on Social Media channels such as Facebook Page, Twitter has fallen significantly
For the above listed reasons and for synergies between different digital media (e.g. Social Media Marketing helps you improve your SEO performance, Email Marketing can help you convert more leads generated through SEM into paid customers), I would recommend you to acquire skills across entire spectrum of digital marketing. In future, you may plan to specialize in 1 or few selective areas.
Check out this article on Career Opportunities in Digital Marketing to get more insights including roles & salaries people get in digital marketing industry.
Hope this helps? Let me know if we can be of any further support.