As much as the introduction and the first two sections motivate the post, I think the last "Don't Just Feel Pleasure, Show It" section is the one that contains the main take-home message in my opinion. If one has benefited by using FOSS, one must acknowledge that it cannot get better and magically sustain itself without support from those who use it or appreciate the principles underlying the movement. The level of transparency and freedom of choice that FOSS offers to users, the DIY ethic and a sense of community it encourages is indeed something to be grateful for, and deserves a generous (subjective) contribution!
If one does not contribute directly by submitting bugs or developing FOSS itself, supporting organisations such as...
* The Free Software Foundation (http://fsf.org)
* Software in the Public Interest (http://www.spi-inc.org)
* Electronic Frontier Foundation (https://www.eff.org)
* Let's Encrypt (http://letsencrypt.org)
... I believe can go a long way. I'm sure there are more - when one feels grateful for the service the software has rendered them, one just needs to make the effort to find out who has written the software and whether there's a "Donate" button somewhere through which this can be expressed, however small the actual amount.