The role of social media in marketing communication is an inevitable topic of modern marketing and this is precisely the subject of this thesis. The development of social media has contributed to the modern way of doing business. Modern business is defined as following/observation/tracking the latest trends and changes. One of the most powerful trends today are social networks. Their advantage is that they are totally timely and spatially limitless. Other advantages are their simplicity of displaying products or services, quick access to information of products or services, direct contact anytime, anywhere, etc. Social media are free online communities, through which users share common interests and activities. This thesis shows exact social networks that play the greatest role in marketing communication: Facebook, Instagram, YouTube, Pinterest, Twitter, LinkedIn, Google Plus. In addition to the benefits of marketing communication through social media, this thesis states and their shortcomings: disclosure of personal data, anonymity, virtual identity, the unreliability of information... The observation method describes an example from the practice of marketing communication on the role of social networks in Donald Trump's political campaign. Finally, monthly data on active users of social media in 2019 has been adopted. Facebook leads with 2,23 billion active users, then YouTube with 1,9 billion active users and Twitter with 333 million active monthly users. Every business entity that wants to be successful in its business must find a way to distinguish itself in the abundance of information through social media, which are the source of potential consumers, users of services and business partners.