If one is represented in social media by others there is always going to be "plausible deniability" for statements and actions. This obscures accountability and induces further chaos into the public realm. No one benefits by such obscuration including the quality of public intercourse. If we can't count on some level of genuine interaction then the value is diminished and motivation for civil discourse is reduced or eliminated.
It's dangerous to concentrate too much power within a single business or institution, whether Google with search, Facebook with social interaction, or Microsoft with computer design. The dangers of monopolization are very real, as they will ultimately begin using that power for evil ends, infringing on consumer rights and possibly price-fixing or otherwise abusing their control. I think it best for there to be a wide market of social media companies as with anything else for maximum competition, employment, development, and options to the consumer. Competition breeds maximal employment and better quality; monopolization is ultimately tied to socialism and inherits the weaknesses thereof. When you have a monopoly, whether from government controlling industry, or allowing business to grow too big apart from anti-trust law, you get a monopoly that inevitably abuses its control and stops caring about quality, efficiency, and accountability.