"not very comfortable for them to talk to strangers face to face (online)" - this may be true of certain people but there are large numbers who will do based on my experience of using a text and voice language exchange site called sharedtalk.com. There are always many people there, particularly seems to be Spanish (especially from Colombia for whatever reason) and Portuguese (almost all from Brazil) speakers wanted to exchange with English speakers. Although the text chat is most popular there, the users are often very willing to exchange Skype names to use voice or video chat.
"how do you motivate strangers to actually teach each other languages while keeping their attention span long enough, say, 5 minutes." - In my experience, you do not actively teach each other the language, more just chat in the different languages and correct and explain the mistakes your partner makes in your native language. Thus, boredom after 5 minutes is not a problem, provided you get along, especially with all the different cultural differences to talk about.