No doubt you have read Ira Levin's 'This Perfect Day'. If you haven't read it yet, you should. It is one of the most unsettling depictions of a dystopia imaginable. At the time or reading, it seemed far-fetched. The radical society which it portrays is still difficult to imagine, but it grows closer.
Most people choose to belong, and most people want to associate with people who do not differ too much from them. This is a natural social impulse. Uniformity of purpose increases the ability of groups to achieve their goals. It translates to an economic choice. This impulse predates mankind.
In the past, uniformity was a geographic phenomenon that was limited to small groups who differed from other groups even though they may have been in relatively close proximity. This can still be seen among the San people today in some places where some bands and villages can't easily communicate with other nearby groups.
The process of cultural assimilation began with the phenomena of war, trade and recreational travel. However the process really began to speed up with the invention of movable type and mass media. The arrival of Facebook (and its precursors) is now having an exponential effect on cultural assimilation. Certain modes of expression, particularly dissemination of pictures and images which require little or no analysis, will have an impact on cultural diversity. Modes of dress and expressive culture, such as styles of music, can be adopted almost instantaneously.
Language, the last major bastion of cultural diversity, is also losing its effect as a barrier.
In this world of global communication in which values are examined and traded across borders, apparent best practices are easy to adopt. This may be a way of dressing, or a type of slang, but it can also extend from foods (for example Asia's adoption of milk and grain in the adult diet) to drugs (hello, ritalin and viagra). The list of shared behaviour and media choices is endless.
The conformity dystopia
I suspect that the primary divisions will be along the lines of Facebook or whatever succeeds it and Baidu in Asia. Additional divisions will arise out of religious fundamentalism and age gaps. The spread of low cost computing in the form of low cost mobile devices will make access to the web almost universal, so although national divisions may arise these will be increasingly challenged by cultural uniformity. Differences will be dealt with by exclusion, initially, however by the process of osmosis, diversity will be greatly reduced. There won't be much room for minorities unless minorities are prepared to exclude themselves. It may be legislatively enforced in the form of firewalls and repressive behaviour towards online presences that 'buck the trend'. However it won't be a benevolent dictatorship that imposes and enforces conformity. We will do it to ourselves.