The correct answer is that it depends on your audience and what day of the week it is. But how do you figure that out?
I'm not going to recommend that you research your audience or any of that; in the end that is all guesswork. What's most useful is to have a graph of when your Facebook friends are active.
That's what my latest Chrome extension, Facebook Counter (UPDATE: defunct), does.
UPDATE: Hi everyone, the big bullies over at Facebook are beating me up. They're claiming that I "stole" their "logo," and therefore my application "infringes" on their "trademark." It was a good run! Please do not send me any further technical support questions.
After installing the extension and leaving a Facebook window open for 24 hours, you get a graph like this:
This graph was from a Friday afternoon to the following Saturday afternoon. The numbers are entirely different on weekdays.
My audience is likely different than yours. Install the extension and see how your own graph looks! Feedback is appreciated as always.
Why can't I view my graph?
First, check whether it is showing a graph structure at all. If so, jump down to the next question. Otherwise you need to make sure that you're copying the URL of your Facebook profile properly.
How do I make sure my extension is working?
Why isn't my graph collecting any data?
You need two things: 1) the Chrome extension installed and 2) an open window with Facebook. If you just installed the extension, make sure you refresh the Facebook page. Leave the page open for a couple of hours - the extension only graphs while the Facebook page is open.
If you are an unlucky Facebook user, you may have to uncheck the Self-XSS blocking on Facebook. Don't worry, the extension isn't getting any data other than your Facebook username and the number of active friends. Savvy users can verify this.
How do I share my graph with others?
Go to https://www.apptic.me/projects/facebook/ (UPDATE: defunct) and search for your graph there. Share buttons are at the bottom.