Social media have become important elements of our information and communication repertoires. They lower barriers of making information of all kinds accessible and of exchanging information with others. At the same time, however, they are not neutral platforms, but they bring in their own organisational priciples into communication, such as the algorithmic personalisation of information flows. This changes the structure of social public sphere and, thus, the prerequisites for the democratic opinion formation. But how are warnings of "filter bubbles" and "echo chambers" to be judged? And do social media really stand for a "participatory netwwork" or rather for new forms of (opinion) power and control? This lecture sheds light on current developments from a perspective from sociology of communication.