Best Practice To Provide Contact Information in Paper
I think this is a good question with only unsatisfactory answers. In short, I don't think you can bank on any method to identify a person (other than maybe the equivalent to a social security number) that is permanent. Neither professional nor personal email addresses are guaranteed to work forever, nor are professional sites like ResearchGate, LinkedIN, ...
Systems such as ORCID, where each researcher is assigned a unique identifier would work, but they will still require some time to gain broad acceptance. This way the paper could cite the ORCID and the researcher can make sure the profile (including contact info) stays current. This system would also make it easier to find publications authored by an individual (especial by somebody with a common last name such as Wang, Ngyen, Garcia, Smith, ...). Of course, there is no guarantee that the ORCID system will last either.