If we compare whether Facebook or Google can be more easily replaced I could argue that Google has a higher chance of being replaced because some startup can come along and build a better search engine for less than 1% of what it cost Google. Whereas I don't see Diaspora really being much different than a Jabber protocol with social networking features. Jabber didn't kill any instant messager protocol, it just further fragmented the instant messager market, which is now dominated by Facebook because of it's critical mass.
You're forgetting that open standards can also threaten Google's entire business model. Semantic Web standards (and related technologies being worked on) will eventually make conventional search engines like Google completely obsolete.
Google is harder to replace because to replace Google, you actually have to replace Google. Develop their search bots, scale up your operation to their scales, sell ads in a market Google dominated, etc. Building a "better search engine" merely gets you a better search engine, you have to monetize it all the way to actually replace Google. This is intrinsically a multi-billion-dollar effort, competing with them the whole way. Distributed internet search has not been shown to be practical, I'm not sure it is even in theory.
Facebook on the other hand can be replaced with open standards and tons of little nodes everywhere. It isn't even all that hard, coordination is the biggest problem. (Or if you prefer, yeah, it's hard, but the original timeline was 20 years. It's not 20 years worth of hard. Again, think back to 1990. Consider the history of open source software since then. Creating distributed-Facebook is not harder than KDE or the Linux Kernel, each of which is only a vanishing fraction of the open source world's output in the last 20 years.)
Semantic Web standards will make Google obsolete? You haven't noticed them leading the charge to actually get these things out into the wild? The little tables of contents, phone numbers, etc in search results? And they're the best in the world, from what I can see, at semanticizing content that wasn't semantically labeled by the originator.
You underestimate Google's own ability to take advantage of new web standards and technologies, and Google's inhouse skills of running huge data centers efficiently. In the pure search tasks, Google is likely going to be really good in the future too.
At one point, the serious future threat for Google was Facebook, because Facebook was a big walled garden, that Google couldn't crawl. If Facebook is the number one web page for a few hundred million people, businesses start to operate inside Facebook more on more, which would make Facebook a good starting point for your searches too, making Google the second choice.
Although other aspects of this scenario have happened, the search part hasn't, and it seems that Facebook has changed the strategy from walled garden to more open place, and try to become a critical infrastructure piece of the whole web with like buttons and instant personalization.
Google will be replaced but not by search engine like better Google.
There will something completely new, like visible now in mass personal computing shift from desktop (Windows) to mobile (it looks like the Android is next main OS).
The great question is what will push Google into oblivion.
Seems like Google's continued ability to dominate will hinge upon whether web browsers remain the dominant mode of navigating the internet, right? Future could be a bit tricky for them, then...
More and more internet activity is taking place in 'closed-wall' communities like apps and Facebook that Google Search can't touch (and, hence, can't monetize) for the most part. This trend is gaining real traction as it seems well equipped to satisfy folks' demand for better, more relevant filters. The power of social networking to help build and refine such filters is pretty telling... just think about how much of your 'news' you get off your Twitter or Facebook feed.
Unless Google figures out a way to penetrate closed-wall communities (via strategic partnerships, licensing of some sort or whatever) or innovate some super useful product that brings in revenue on the scale of AdWords (GoogleMe?), we'll probably see a piecemeal dethroning happen.
This is probably one of several reasons why smart people are migrating from Google to Facebook.
You're forgetting that open standards can also threaten Google's entire business model. Semantic Web standards (and related technologies being worked on) will eventually make conventional search engines like Google completely obsolete.