Google may reduce its commitment to open source. Google has contributed to the open source world in multiple ways, releasing major products as open source, financing work on free software projects through its payments to Mozilla for search referrals and its Summer of Code scheme, employing several top hackers at what are presumably appropriately generous salaries, and building its entire computing infrastructure on top of open source as a demonstration of free software's unparalleled scalability. However, there are areas of concern, notably in the semi-open nature of Android and the delay in releasing its source code, and the poor way the Android market (aka Google Play) is policed for viruses. The increasing importance that Google evidently attaches to Chrome, and its rapidly rising browser market share, is also a cause for concern, especially for free software alternatives such as Firefox.
Microsoft may turn into a full-blown patent troll. Although Microsoft has been a thorn in the side of open source for over a decade and half, its attacks have so far been badly misjudged and ineffectual. However, thanks to the deeply-flawed US patent system, there remains the threat that its current patent assault on companies using Android for their products could be widened to directly include open source. Given the absurdly low bar set for patentability in the US, and the thousands of trivial and/or obvious software patents that have been granted there, it is quite probable that at least some of Microsoft's patent hoard would read on open source code. Although many of those software patents might be invalidated for reasons of prior art, there remains the risk that a few might withstand all attempts to have them reviewed and struck down. Some might even be for crucial techniques that have been in use for decades, which would make coding around them difficult.
The good news is that all these "risks" are relatively low – a bit like the chance of those earthquakes wiping out Redmond – but, like the earthquakes, they are something to be borne in mind. Just as Microsoft presumably ensures that its buildings conform to the latest anti-seismic standards, so open source needs to put some effort into preparing for the key issues touched upon above.
That is, despite open source's unparalleled success in the lower levels of the stack, it has still not conquered the hearts of ordinary users in the way that Apple, say, has. Canonical, among others, is seeking to address that, but more work is needed. The rise of Facebook may be harder to counter directly – unfortunately free alternatives like Diaspora are unlikely to catch on simply because of network effects working against them. The good news is that people are fickle about social networks – remember how MySpace once looked indispensable and thus invincible? So it's quite likely that Facebook will simply become less popular, especially with younger people who will have moved on to the new new thing.
Google will probably remain an ally of open source not least because it has garnered so many enemies recently – Apple, Microsoft, the European Union, etc. – that it needs all the friends it can get. As far as Chrome is concerned, that's partly a matter that Mozilla can solve on its own by addressing some of the issues that users have with Firefox and Thunderbird.
As for Microsoft, the key there is to fight tooth and nail against software patents. In theory, that battle was won in the European Union back in 2005, when the European Parliament overwhelmingly rejected the introduction of patents for software. But the danger is that the proposed European Union Patent could allow software patents in by the back door, enabling them to wreak the same havoc in the EU as in the US. This means the threat from Microsoft's unspecified patents remains real, even in Europe – and is definitely something to include again in next year's 10-K Risk Factors for Open Source Inc...