what leaves me skeptic about these kind of alternative services like GNUSocial and Diaspora is not much when, if ever, or how they will be able to attract the necessary critical mass of users that makes them relevant. Instead, it what would happen after that. The main two questions I would be interested in would be:

(1) do they scale to the size of, say, Twitter (~400M Daily Active Users, Facebook ~1B DAU)? (2) what would be their business model?

The thing is.. hypothetically GNUSocial is very nice. It's free software, it's an open protocol, you are supposedly in perfect control of your personal data. In theory, there's no reason not to prefer it to the current state of things (if only you could make it appealing to your grandma or 14 years-old cousin).

However, when, for instance, the Justin Biebers, the Lady Gagas, the Obamas join it together with their hundreds of millions of followers, or simply when the tweets per minute reach hundreds of thousand or even millions, can the GNUSocial protocol ensure a reliable and responsive (~ real time) service? And if it does, what's the maintenance and hardware cost for the confederations? And how are they supposed to financially cover for it?

Sure, there are services based on open protocols that have been extremely successful at scaling (e.g., email, or bittorrent), still, they have different requirements than a social network. Can we make an open social network of independent confederations scale? Probably yes, but whether GNUSocial is ready for it or not is an open question (as far as I know..).

In any case, take the example of the email. The protocol is open, there's a "confederation" of independent providers taking part to it. It handles a massive amount of data every day. You don't even have necessarily to store your data on a cloud server. That's great.

Still, at the end of the day everyone uses free mail services (GMail, Yahoo, you name it), heavily maintained by hundreds/thousands of dedicated (and skilled, and expensive) engineers, storing gigabytes of your data (backed up and redundantly replicated over different geographical areas), and financing themselves using targeted advertising. Sure, you can pay your own, trusted, mail provider an annual fee and have (apparently?) complete control over your data. But how many people actually use these services outside of the work environment (where your employer actually covers the cost)? And I'm not talking about the community of hackernews readers. I'm talking about your non-techy friends, your relatives, the random guy/gal you meet at the pub, that is, the people that ultimately are necessary for your service to reach a critical mass.