In any case, it has at least replicated the del.icio.us functionality, and then added more, such as archiving page contents. The tags are still there too, and it prompts with other users' tags when you add a bookmark. Oh, and an API, which is very useful for programmatic use of the data once saved.
If proper free accounts with all functions are supported, then advertising is a likely revenue model.
To combat this problem, I've added sharing over email, SMS and Slack to my project, which is a personal search engine and document manager. There's no reason to not allow a very basic and free account function to read these shared links, without a lot of auth and registration getting in the way. The service already has the guest's email address, phone number or Slack account from the other user. This can be used to send a token for easy login. Limits would be added so these accounts can only store a few URLs themselves unless they upgrade.
This keeps privacy protected, hopefully.
- Looked at "popular" bookmarks, e.g. https://pinboard.in/popular/ - At the far right end of each page title, there's a little number. For example, beside "Lotus 1-2-3 For Linux", there's currently the number 21, which links to https://pinboard.in/url:61d59935774d1affe22713a7423c78bd17ef.... - Following that link, you see who bookmarked that link, with name, tags, and some related other links. Of course, clicking on a user name will show you other things they've linked, at least using the default public setting.