Rails was a horrible technology choice to to build an open decentralized messaging protocol. They were doomed from the start. NodeJS would be a better choice now.
Are there really that many developers clamoring to build new apps on yet another closed, for-profit, centralized social network with no users? I sure don't. I'm already done with Twitter and Facebook apps, the centralized model is simply not the way forward and I can't be alone with this opinion.
Techcrunch posted an editorial a while back that I agree with:
Can you expand on why you consider Rails to have doomed them, and why NodeJS would be better? I'm not particularly familiar with either technology (besides reading about them here on HN), but it's my understanding that Rails powers some very big websites, like Hulu and Github.
That said, there's already a working Twitter-like distributed network. I hard my own StatusNet node running on my personal server and getting messages from people on Identi.ca, until I realized that I found the idea of the platform interesting, but not the platform itself (nor Twitter, for that matter). The fact that half the accounts were abandoned didn't help either.
Rails is geared for CRUD apps, not messaging. Whereas NodeJS is built for messaging. Picking the wrong technology stack for what needs to be a messaging protocol that resembles SMTP definitely contributed to Diaspora's lack of adoption, but certainly wasn't the only reason.
StatusNet actually sounds kinda good. Why isn't it more popular? I'm not sure, perhaps people need something more than just a replacement for Twitter. I think something that will eventually become successful will be some sort of multi-use low-level protocol that handles many types of "social communications", for example: Twitter + WordPress + Private IM + Voice Message.
Maybe you have deep enough experience with Node to avoid the thin ice, but my experience has been one of sparsely-featured and buggy libraries when compared to their counterparts in Python, Java, etc. All languages need time to grow up, but for now we use Node as a pinch-hitter in our stack (I created a Snowflake server with great results)
Honestly, Facebook has a lot of frontend code. I don't think Rails is a bad choice for that. My experience with large systems is that they're usually not homogenous. There's no reason they couldn't implement the Rest API components outside of Ruby if that was their biggest obstacle to success.
Though I'm not sure it is, because even though you've obviously given this issue some thought, you didn't really articulate who was put off by their use of Rails?