Other than the police, It is more likely that someone could identify you by seeing you pop up on different web forums with the same or similar user name. For example your employer, thinking they recognise you on a forum and perhaps also knowing your hobbies and other interests, could look in such hobby websites for the similar user name, profile and style and so confirm (at least to their own satisfaction) that it is indeed you.
There is nothing stopping me as a user doing the same thing though - IE, take on someone elses personality. But I agree - many people use the same username on many forums.
As for Facebook - forget about any privacy. Employers (or potential ones, or anyone else in the world) can look there as a matter of routine. FB is unusual for a web forum in that it is the custom to give real names, and would be the starting point of any investigation into you.
Although you can lock it down a fair way.
It's just many people don't bother.
When I worked for an employer, I didn't feel the need to link my facebook profile to them under the "Employer" section. But then many people could guess where I worked if they looked at my friends and found the common theme between them I suppose.
A bit stalkerish though, and I doubt TOCs employ people to delve too deep into that sort of thing.
Just saw on Facebook a case of a guard who has (allegedly) been sacked from his TOC, because someone in the same group was screenshotting his posts about encouraging users to put in complaints about service performance, then sending them to 'management'.
This is where the digital age cannot be trusted though.
I can easily take a screenshot of any post on this forum and make it look like the user said anything I want. Yes, the "original" will still be here, but it could be suggested you edited the post after the screenshot was taken. You'd just need to take a screenshot of an already edited post.
It's where things are becomming difficult to provide, such as bills for ID/mortgage purposes with so much going online.
Even a downloaded PDF can be changed with minimal effort.