Publons - is it a serious thing?
Here is their current web page, which in contrast to the Google+ page you linked to appears to be up-to-date.
Although this is the first time I've seen this site, my impression is that it's ethically okay and not a hoax. That said, it also seems quite unnecessary to me, and I would not bother to use it.

What will it yield scientists in the long run?

--  I would estimate:

With about 2% probability, the website will catch on and become popular. Probably this would be at the insistence of university administrators, who are often looking for new ways to measure and evaluate the performance of faculty.
The website will yield another hoop for researchers to jump through, and possibly (as the website claims) to increased recognition of researchers who do an unusually good job of peer review.

With about 98% probability, the website will largely be ignored, and the net effect on the scientific community will be nil.


In general, I would advocate ignoring advice originating from commercial publishers; personally, I would only consider using this site  if I were urged to do so by scientists in my field or by administrators at my university.