The reasons why the number of sourceforge visitors with Linux is declining might also include GitHub, which became hugely popular in this crowd.
The decline in Sourceforge downloads by Linux users is likely to be down to the fact that most will simply grab the package from their distro's repository, rather than getting it from SF. If I download, say, FileZilla on my Windows machine, I get it via Sourceforge. On my Ubuntu box, it's in the repo.