I strongly disagree. FOSS is a huge force for both backend development and front end web development. One can also argue that Android is mostly open. EDIT: The large majority of servers in the world are running on FOSS. I can also predict a large number of upcoming devices like the internet of things to be running on it. Whether or not they run Android is another story.
The only reason FOSS may not be a "player" is because it's not a single monolithic organization, but instead it's a very loose coalition of thousands of individuals, organizations, and companies.
> How many languages can you use to write apps for your iPhone or Android device? Objective C and Java respectively and that's about it.
This may be the case for clients, but a lot of mobile apps also depend on server side apps.