In comments on a post on G+ I said something I might regret:
"There are plenty of RDF-based applications around, but none really have much broad public appeal."
Ade Oshineye responded with "why do you think that is?"
Ok, overnight I remembered there's at least one app (or set of apps if you prefer) that uses RDF and has a lot of adoption: Drupal. According to Wikipedia it's used on at least 1.5% of Web sites worldwide, and has RDF in its core. Then there's data.gov.uk, a public-facing national government site that's RDF through-and-through. I'm a little out of touch, there are no doubt quite a few other good examples of where I'm wrong.
But given that RDF has been around for 5 years*, it's the way of doing data on the Web and virtually every Web-oriented app uses data somewhere, why isn't it ubiquitous?
(* solid specs came out in 2004 although SPARQL wasn't until 2008 so I'm splitting the difference for a rough date for when it became usable)
RDF isn't something that's going to be in your face anyway, so "broad public appeal" is slightly off-target. Developer adoption may be a better key. Whadever.
In terms of it as a database tech, compared to relational DBs (MySQL etc), custom data handling (Twitter uses Ruby message queues), novel DBs (Facebook uses a key-value store Cassandra apparently) RDF stores don't get much of a look-in. Ok, arguably the big scale things need to be custom to hone performance, but why, alongside the Big Data handling, don't we see RDF augmentation?
For consuming apps and desktop apps, I can't actually think of any well-known ones off the top of my head (I think quite a few of the music apps on Linux use librdf under the covers). I don't have a mobile device - any iPhone apps?
What I find a little bizarre (and please give me counter-examples), is that in the areas where RDF really shines - Web-oriented data integration and reuse - there are hardly any well-known apps out there at all, using any technology. There are a handful of feed aggregators and things like techmeme, but the level of integration there is pretty trivial. (Before Kingsley jumps down my throat - OpenLink Virtuoso is seriously good at this kind of stuff out of the box - but what I'm after is where these things are being used by twitter-sized demographics).
There's certainly something to what Lee Feigenbaum said the other day, the wrong question is usually asked, it should be: What can I do with Semantic Web technologies that I wouldn't do otherwise?
In terms of app-building, right now most parts of most things can be built relatively easily using other technologies, so unless the RDF stack is part of the developer's on-hand toolkit (like e.g. LAMP) it won't be first choice. I do suspect that while the false perception that RDF is complex per se isn't so prevalent these days, there's still a notion around that RDF is complex for the benefits it offers. i.e. linked data isn't perceived as a significant value-add, so why bother? The primary objectives can be acheived by pushing around little JSON objects ("jobbies"?) in a fairly arbitrary fashion, so why look further? But data on the Web surely isn't a niche thing...
The other day I posted a question on G+ that probably should have gone here: All the necessary components were in place for online social networks, in a distributed form, before Facebook & co. came along: blogs, aggregators, the various protocols. So why were Facebook & co. so successful? (got some good comments there, and was very pleased to find out Andreas Kuckartz is researching the question)
The question of data on the Web seems to lie in a similar socio-politico-technical morass. On federation, I'm afraid I'm inclined to agree with Eric Siegel : "I predict decentralization is inevitable, but its very very far away." I feel pretty much the same about the Web of data, though perhaps not so far away (unless I'm confusing small and far away :)
[ooh - a good point on that from Seb Paquet I'd missed before: The folks who grokked decentralization didn't master social experience design and UI design as well as Zuck, and decentralized infrastructure is harder to monetize so getting funding was difficult.]
If RDF is so great, we should all be rich by now? :)
Another quote, it must have some relevance - via the BBC, from Sir William Preece chief engineer of the British Post Office in 1876: "The Americans have need of the telephone, but we do not. We have plenty of messenger boys."
Still no system here yet, comments to G+ again.