What specific sharing are you talking about, the wavelength, the individual strand, the entire cable, the trench?
Leased fiber services are generally lit by the provider and may carry other customers' data on it, too. Albeit, this is still relatively secure, customers cannot see each other's data because the wavelengths are filtered.
I'm not sure why this Wikipedia link was submitted without question/comment, so I flagged it and encourage others to do the same.
This is one of my pet peeves. I wish OP (in general) would comment as to why they're posting, what they found interesting. I don't flag these posts, but they do sometimes feel like a lazy karma-grab.
I have others which are fibre strands I pay another company to manage. They emerge as a single fibre at my location, and shine a powerful enough torch down and it comes out the other end. I'm sure that those strands go down cables with a few hundred strands on at some point though. These are mainly metropolitan range, but I've got a couple in the 100km+ range.
My company has some which are wavelength services, these are multiplexed at local exchanges and I assume the wavelengths might be shifted. I don't deal with these. In theory I guess I can shine a torch at a given wavelength and it pops out the other end
And then I've got ethernet circuits which are managed, I get an ethernet handoff at each end, but at its core it's just a packet service.
All of these can be tapped by advesaries, it depends on your level of paranoia. Best to encrypt.
The first 3 you can run say jumbo ethernet frames over. Or you can run non-ethernet traffic if you want.