Should I expose a user ID to public?

If it were dangerous, Stack Overflow wouldn't be displaying user IDs in their URLs in order to make user profile lookups work: https://stackoverflow.com/users/104826/rfactor

Edit of seriousness of immense levels: if user IDs are themselves sensitive data; for example your primary keys for some reason happen to be social security numbers, that'll definitely be a security and privacy liability. If your user IDs are just auto-increment numbers though, you're clear.

Generally it's not a problem but it can give away hints on how active your site is, like how many users you have etc. If you consider this sensitive information or maybe even good marketing is completely up to you.

There's a story that this was one of the reasons the germans lost the WW2. They had sequential serial numbers from production written on each tank. By collecting id numbers from tanks taken out the british could estimate how many tanks the whole german army had and make new strategies from that.

I have found that exposing primary keys that identify physical entities can create headaches.

Imagine if two blood samples come into a laboratory and test results are generated for each sample. Many different kinds of test might be done and each record representing a test result will have the sample_id as a foreign key.

If you share the database ID with the customer and you discover that two samples were accidentally switched, you will have to update the foreign keys in all the detail records representing the tests. If you instead exposed some other unique name outside your system, you will just need to switch the two unique names on the sample records in the master table.

There are other advantages related to data migration and there are advantages when entities are represented in more than one database in which it is difficult to create records with identical database ID's.

In my experience it is always best to expose a unique identifier other than the primary key outside your system. It gives you more flexibility in resolving data mix-ups, dealing with data migration issues, and in otherwise future-proofing your system.