symbolipoint said:
One main reason I started the question is that some people have been away from a former career for many, many years, and hope to look for a path back to it. No current contacts. No current references.
I can't address your primary question regarding new social media. But as someone who switched careers multiple times and, on occasion, considered returning to a former career, I'd like to give you these general comments:
(1) A lot depends on what career you are talking about, and how long an interval is "many, many years". If you're in a fast-moving, high-tech area, your expertise can be stale within a year or two, and obsolete within five years. So you need to seriously reflect on what value you can offer to a potential employer.
(2) If you have no personal connections to potential employers, then you are likely to return to a former career only:
(a) If there is a demand for expertise in legacy products or systems, and you happen to have that expertise. The primary example of such a demand was in the latter half of the 1990's, when IT departments were faced with the potential Y2K meltdown. If you had a previous career in computer programming with expertise in COBOL, and had moved on to another career, you would have had no problem returning to a career as a COBOL programmer (although for several years at most). There is a niche demand at times for people with expertise in older technology, such as vacuum tubes.
OR
(b) If there is an overall shortage of skilled people in a particular field in which you had prior experience. E.g., optoelectronic devices for telecommunications was a sizzling field in the 70's and 80's but got quenched in the early '90s; many were laid off and left the field. But by the late 90's, demand suddenly heated up again due to the Internet Bubble, and there weren't enough recent grads to fill the demand ... so people with the right background were welcome back even though they were not up-to-date on the latest technology (unfortunately those who returned were likely laid off again just a couple of years later when the Internet Bubble burst).
(3) Otherwise, your best bet for returning to a former career is via former colleagues who worked with you. Colleagues who will overlook your out-of-date knowledge (but are confident that you can come up to speed in a reasonable time), because they value your generic skills: initiative, problem solving, communications, working in teams, leading teams ...