The unfortunate fact is if a user who views what you post is using a windows machine, the likelihood of the information on their screen being captured by Microsoft is overwhelmingly high.

I guess you may have to approach the issue how you would the public-facing internet at large: if you cannot verify who and how people are viewing your material, do not post any material that can be accessed by windows. If you must, post it through a trusted circle of users who also understand the issue.