A license (such as Facebook's) that prevents me from redistributing a modified version could still qualify as "open source" so long as simple mirroring wasn't restricted.
> In practice, open source stands for criteria a little looser than those of free software. As far as we know, all existing released free software source code would qualify as open source. Nearly all open source software is free software, but there are exceptions. First, some open source licenses are too restrictive, so they do not qualify as free licenses. For example, “Open Watcom” is nonfree because its license does not allow making a modified version and using it privately. Fortunately, few programs use such licenses.
> Second, and more important in practice, many products containing computers check signatures on their executable programs to block users from installing different executables; only one privileged company can make executables that can run in the device or can access its full capabilities. We call these devices “tyrants”, and the practice is called “tivoization” after the product (Tivo) where we first saw it. Even if the executable is made from free source code, the users cannot run modified versions of it, so the executable is nonfree.
> The criteria for open source do not recognize this issue; they are concerned solely with the licensing of the source code. Thus, these unmodifiable executables, when made from source code such as Linux that is open source and free, are open source but not free. Many Android products contain nonfree tivoized executables of Linux.
http://www.gnu.org/philosophy/open-source-misses-the-point.h...
(Pasted the most relevant portion in case you lack reading skills.)