Researchers and bloggers and social media(ers) have tried to measure the best times to post on blogs, and Twitter and Facebook. Why?
It’s important to know when you are most effective on your social media sites. If you want people to see your posts, you should submit them when the greatest number of individuals will see them.
Most studies show that posting before 8am and after 5pm will generate the most viewers per post. Twitter and blogs are easier to measure. How do you measure when to post on Facebook? How frequently should you post, and what’s the best time of day?

Jeff Widman found the answer to that question: “Post whenever the most recent status update for your page stops showing up in your fans’ News Feeds.”
How do you know when that happens?
Each post on Facebook is unique. Links and pictures may have a longer lifespan than status updates, and vice versa. You need to calculate the behaviors of posts and come up with an average for each category: photos, videos, links and updates.
Here is his suggestion:
This method should give you a pretty good idea of how long your posts last in your follower’s news feed and when your followers are most active. Once you’ve collected the data, feel free to experiment a bit to see what works best for you or your company. Lastly, form a method, if you can calculate the most popular time to post, always post on that day of the week or that time of day. If comments and impressions begin to drop off, re-calculate and try again.