Recent buzz about Google's OpenSocial and Yahoo's Inbox 2.0 is pretty fascinating stuff. The convergence of email and social networks hits upon a number of sociological issues. My thoughts were spurred by a thoughful article on TechCrunch.
Stepping back from the Internet to consider modern history a bit, take the post-modern transition from nation-state consciousness back to new or renewed kinds of "tribal" associations. For example: the Balkans, ex-USSR, Blue states vs. Red states, Apple vs. PC.
Most people in traditional nations had a relatively stable sense of social identity, with relatively low social barriers between strangers. Enter the new tribalism - now the differences between a stranger and a friend are quite striking; issues of trust and authority change dramatically, especially in a social context, but tesselate throughout the political economy.
The older institutions of nationhood like (democratic) government, broadcast media. and corporations feel the impact and are scrambling (strategically or tactically) to respond with varying results. Recent particular examples are how Europe has shifted to ethnonational conservativism, and South America to class socialism... both domestic reactions to globalization of one form (immigration) or another (privateering).
Getting back to E-mail (Yahoo, Google) vs. Social Networks (MySpace, Facebook), a similar old/new analogy seems to apply. E-mail can be sent by anyone to anyone as long as you know the address - remember the old days how it was such a rush to get e-mail from a stranger. Richness of experience was defined by the level of novelly accelerated (and free), and narrow, contact. Along came spam - now, even filters, then folders, and finally tags, still barely helps stem the unwieldly tide.
Social networks define the internet experience (through who you know) as "Connection", not "Contact". In fact, the semantics work - you connect without needing to make contact. Richess of experience is level of connectivity - with transmission accomplished through a sort of inverted broadcast.
So.. E-mail Nation feels a need to redefine itself to match the impedance of the evolving network. Leveraging the huge asset that is email makes some sense. It always used to intrigue me - since Yahoo knows who I email, and how [in]formally with what frequency... they must be trying to monetize that intimate model of my.. um.. social network. Is it just good old-fashioned privacy policies and culture that prevented acting on it meaningfully until now?
At this point, since everyone is explicitly providing sites with personal connectivity and tagged at that, the culture seems quite different from 10 years ago. I think Google bootstrapped old-school search+email with the strategy all along to go deep tribal with tags/adwords and something like opensocial in mind.
I think Yahoo seems to struggle competing because of its old-school inertia. Of course, now social sites have their own spam problems and privacy issues. Technology continues in its race condition with culture, and so the landscape is likely to change too much to predict a "winner". Check out this comment.