I am sure there is some kind of report out there or company who has tracked the sites' usage and when they are getting the most traffic, although maybe that is illegal or maybe it just costs a lot of money to access. If I had to guess, though, I would say at a convenient time for whichever place has the most subscribers, since the sites are very global and occupy many time zones.
You should post to Facebook or Twitter at the time when you expect most people to be online. But since you may want people in different time zones to see your content, you may need to share the same post several times but ensure that you do so when you expect people in that time zone to be online. This I believe would be a good way to reach more people via social media.
I think it's the weekends and holidays where people are mostly on Facebook and Twitter. Though I guess after work and during lunch breaks people are also busy doodling away on their gadgets. You have to know when people are mostly online based on your observations because people in one part of the world has different time settings from people of another part of the world.
Like many others have said that evening times tend to work best each time zone. I post about 30 mins to 1 hr before peak times to get the best results. Again it all depends on who your target audience is. What age group and what main niche are you going for? Giving us this info would help out a lot to give you a better answer. I have found that evenings work best for me in the travel and health niche. After 8pm has worked best for me in the weight lost niche cause this gives time for people to get home from their workout at the gym and are still in the lose weight mental state.
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