If you havent see it already, check out the links in shor's comment below - there are some great resources in there. In some cases you can also consider surveying your current audience or customers through email, on-site surveys or SurveyMonkey. Be sure to ask for some profiling information that you can use for determining specific persona needs like age, sex, location, etc. (Probably best not to make it sound like a creepy text chat like I just did though...) :)
Hey Ted, thanks for the great questions! The peak times refer to your particular time zone, if you are targeting an audience that resides in the same zone as you. You can also use tools to find out when most of your audience is online. For example, Facebook has this built into their Page Insights. For Twitter, you can use https://followerwonk.com/. Many social posting tools also offer this functionality.