I've come across these kinds of prototype link sharing sites in my battles with SEO spam.
On the face of it SEO spam is simple: the bad guys bung a load of links into your wiki or blog comments in the hope of gaining google rankings for their "SEO clients", but...
Increasingly they started bouncing links all over the place in complex spamdexing webs. Link sharing sites often get thrown in the mix, along with cross-linking other blogs/wikis where they've got their spam to stick. This all makes it easier to evade any filtering by a particular blog/wiki admin, but maybe also makes it harder for google to filter and down-rank the baddies, and finally depending on how complex a spamdexing web is, it offers protection to their clients because it ends up being impossible to see which end websites the spammer is actually aiming to promote (the links tucked away amidst the randomised cross-linked chaos)
But maybe that was a game of a few years ago, and now spam bots are mostly just trying to push porn on social media.
I'm facing the same issue with a URL analysis service I operate which lets you take a snapshot and screenshot of a website. I get countless submissions to other sites which allow user-generated content (Reddit, Medium, random support sites, everything that's "open") and all of these sites themselves host images and links to "Watch XX Soccer Game Live 2020 HD" etc., so all link-spam. I found my ways to battle these submissions and users, but obviously don't want to reveal them here.
The other thing is signup-spam. I'd say a full 50% of my signups are spam, and I do my best to remove those user accounts. What surprised me was that the spammers seem to be human, i.e. using a Gmail address (which requires verification), solving Captcha, entering form fields, clicking the email-verify link, etc. Just crazy. Again, the countermeasures are not something I would want to publicize...
But maybe that was a game of a few years ago, and now spam bots are mostly just trying to push porn on social media.