http://www.stallman.org/stallman-computing.html
I firmly refuse to install non-free software or tolerate its installed presence on my computer or on computers set up for me to use.
However, if I am visiting somewhere and the machines available nearby happen to contain non-free software, through no doing of mine, I don't utterly refuse to touch them. I will use them briefly for tasks such as browsing. This limited usage doesn't give my assent to the software's license, or make me responsible its being present in the computer, or make me possessor of a copy of it, so I don't see an ethical obligation to refrain from this. Of course, I explain that they should migrate the machines to free software.
Likewise, I don't need to worry about what software is in a kiosk, pay phone, or ATM that I am using. I hope their owners migrate them to free software, for their sake, but there's no need for me to refuse to touch them until then. (I do consider what those machines and their owners might do with my personal data, but that's a different issue. My response to that issue is to minimize those activities which let them get any of my personal data.)
That reasoning assumes I was not responsible for setting up those machines or for how it was done. By contrast, if I were to ask or lead someone to set up a computer for me to use, that would make me ethically responsible for its software load. In such a case I insist on free software, just as if the machine were mine.
Skype is another kind of exception. Using Skype to talk with someone else who is using Skype is encouraging the other to use nonfree software. So I won't use it under any circumstances.
I do not use social networking sites. They are inherently inconvenient for me.
I do have a twitter account called rmspostcomments. I use it to log in on some web sites to post comments on articles, but I do not post on twitter. Any other twitter account that claims to be mine is an impostor.
The rms account on identi.ca repeats the political notes from this site, but I do not post on it directly.
Aside from those two, any account on a social networking site that says it is mine is an impostor.
As for Facebook or Google+, I reject them on principle because they require people to use their "real names". I am proud to identify myself when stating my views, but I can afford to because I am in a fairly safe position. There are people who rationally fear reprisals (from employers, or bullies, or the state) if they state their views. Let's reject any social networking site which has that requirement!
Of course, Facebook is bad for many other reasons as well.