Since they invested their time in you, I would suggest that you give back in kind. Call them, send them a note, email/text, etc. It is easier than ever to keep in touch with people now-a-days. Meet them in person if they are open to that and you are able to.
If some of these mentors are retired, I'm sure they would love to hear from a former protege/student/mentee. Watching my dad after his retirement, he always enjoys these interactions with his former colleagues -- talk about their profession or department. Made his day/week in most cases. Retirement can be professionally lonely sometimes.
Interesting hypothesis. If it is true, what does it say about social networks like FB if in-person networks provide better trust building than online ones.
If some of these mentors are retired, I'm sure they would love to hear from a former protege/student/mentee. Watching my dad after his retirement, he always enjoys these interactions with his former colleagues -- talk about their profession or department. Made his day/week in most cases. Retirement can be professionally lonely sometimes.