Before choosing the best time of day to post on social media you should analyze your target audience and time zone.
Here are some general suggestions:

Facebook: 1-4 pm and 2-5 pm weekdays.

Twitter: 1-3 pm weekdays.

Pinterest: 2-4 pm and 8-11 pm weekdays with weekends being the best.

Instagram: 5-6 pm weekdays and 8 pm on Mondays with a sweetspot at 6 pm.