I would like to fulfill my service requirements for the tenure review process. My question: Is it possible to help fulfill these requirements through on-line activities? E.g. Mentoring students online, or participating in some sort of on-line collaborative community?
@keshlam's comment is correct: this is a question for the folks in your department. There are activities (internal and external) that feel like "service" that are not viewed as "service" ... it varies by department and school. Just like expectations for research and teaching.
And make sure you ask folks who are giving you fact-based, not opinion-based answers. You should have a tenured faculty mentor (if you don't, get one!), but they might not know the facts on this. There should be an RPT chair in your department or school: ask them. You might also ask your chair: if they chair views what you are doing as reasonable service, they may be able to argue for it counting, even if it's unusual.
But, in the end, this is not a question for anyone outside your school. We don't vote. :)