We all know monopolies are bad. We even have laws against them that sometimes get enforced. However, today we have new kinds of monopolies that affect us without us even noticing them for what they truly are. And technology plays a central role.
When we look at social networks we see them usually as a single market, maybe divided between full-blown ones and microblogs. Thus we see Facebook and Google+ competing on a single market that seems to be divided between many players, including a small slice for Diaspora, for example.
Competition only works where there is a real possibility to choose a product or service. For example, competition between family car makers works, because customers can actually choose different family cars and yet be able to travel to the same places on the same roads, and using the same kinds of fuels.
Similarly, competition in areas of web browsers and e-mail providers works because regardless of which web browser you choose or with which e-mail provider you set-up your account, you will be able to access the whole web and to contact users of all other providers.
This, however, is not the case with closed social networks. Facebook users cannot contact Google+ users and vice-versa. Technically, from users' perspective, Facebook and Google+ are actually separate markets, each of those with a single monopolist provider (Facebook and Google, respectively).
Once we start seeing technomonopolies for what they are, we can start exploring their consequences, in the same terms we consider consequences of any other kind of monopoly on any other market.