Unfortunately this is not a one-size-fits-all answer. The best way to find out when your audience is active on social media is to ask them. Alternatively you could guess based on what you know about their lifestyle.
If your target audience is young stay-at-home mothers they are unlikely to be checking their social media first thing in the morning when they are trying to get kids ready for school, or anytime from 3pm (school pick up) to 7pm (bedtime – hopefully), but they may be active mid-morning, at lunchtime or later in the evening.
If your target audience is busy executives they are unlikely to be surfing Linked In first thing on Monday morning, when they have a pile of emails to go through, or on a Friday afternoon when they want to get finished and go home, but they might have time for a browse on Tuesdays at 10am or Thursday lunchtimes.
The final issue with social media timings are that you may end up scheduling posts whenever you have the time….solution to this is to use a scheduling tool (such as Hootsuite) to make sure posts go out at the best time for your audience, not for you.
Many of these tools also have a Auto Scheduler function, which in theory can choose the best day and time for your post – use with caution though…