Question: I was recently speaking with my manager, who stated that while my work is satisfactory, they've been noticing I'm spending a lot of time of Stack Exchange. I find Stack Exchange to be very informative and helpful in my work.

Particularly considering this Stack Exchange site has many users who spend a large portion of their time on here, how can I explain to my manager how useful Stack Exchange is for me?

Answer: 
Looking up answers to work problems is one thing, but answering questions, commenting, etc. are really just a form of socializing which your employer has the right to say "do it on your own time".

Sure, you're creating value *in general*, but if it's not directly related to your job, then it's not really justifiable.
1. **Don't log in.** The value added at work for *reading* StackExchange sites *far* outweighs the value in answering questions.
2. **Only visit "hard" SEs that you need to get your job done.** Workplace.SE, bless its heart, is not one of them.

You may learn a lot from reading programmers/sofware engineering, workplace, etc. and it may help you grow in your career. But if you are having friction with your boss I would strongly recommend taking perusal of these sites to your own hours. Writing it off as career development is not an argument that is going to make you look good if your boss feels the need to bring it up to you.

**Take your job really seriously right now.** Your manager gave you some pretty serious signal that you may be being unproductive. That was nice of them. Don't wait for them to escalate this into a performance improvement plan.