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: 
If you are using Stack Exchange sites to find information that helps you complete work-related tasks, **document** the occasions when you solve a problem based on information that you get from an SE site.

For example, if you are responsible for tasks that are tracked with some kind of bug tracking, ticketing, or version control system, and you find yourself cobbling together a solution from a set of Stackoverflow or Superuser posts, record the URLs to those posts in the notes on the ticket. (You should be doing this anyway for completeness in tracking the issue). **Then, you should be able to easily report a real number of instances in which SE has directly provided value to you and your company.**

If you are building and expanding your own knowledge base, yet still able to apply the knowledge to work-related tasks, use the conversation as an opportunity to talk to your manager about continuing education. Be specific, and request training around topics and technologies that directly apply to your work and your firm. You may find a way to get additional training and have the company pay for it, in which case, your continuing education time would be sanctioned.

If, however, you find yourself researching subjects that simply interest you, or that you wish you were working with but aren't because of, e.g. company culture, you may want to ask yourself if you are growing in the right direction at your current job. In this case, be honest with yourself about how relevant your Stack Exchange research really is, and whether your manager's advisement might be a good reality check.
So there are two possibilities here - either your boss is over managing and just want to show you that it's he who is in charge here and you should obey or - or - he is not satisfied with the quality of your work. 

There's no other option in first case than to leave - never waste your time working for non-professionals. But to be honest with you the very fact you had asked this question and the way you asked the question hints that you assume that your boss is a reasonable person who can be convinced in something if he was wrong. That said, we don't investigate the first possibility here. 

In second scenario there's only one way to convince your superiors that you are doing it right - concentrate on your work. Words like "we are more or less satisfied, it's just that you spent too much time surfing Stackexchange" could be a sign that they are actually not satisfied and politely indicating this. Because actually good boss not that much care what and when you are surfing in case the work is done. 

Here's the strategy I suggest to stick to:

* Sort tasks you are going to work on today by how easy you can solve them without Internet, relying on your own knowledge, do the easiest ones first.
* Don't surf Stacks, don't read interesting questions and answers even if they are related to you work in that sense that they are making you a better professional overall. Don't answer, don't comment. Just google an answer to your particular question and, as soon as you've found it, paste it to a text file or (even better) your corporate wiki.
* Make a habit to first look for solutions in that local storage I've mentioned earlier. You'll be surprised how many routine repeating stuff can be saved and easily find in a simple text file without any googling at all.

I encourage you to start with these steps and trust me, you'll see how your work relationships will get better.