Knowing peak traffic times on certain social media sites helps too! I've been using this to track peak traffic times for Tumblr. I think it's helped, though I have no data to back it up. Might be time to start using bit.ly!
And Instagram was fairly easy to figure out. People (myself included) tend to check Instagram at work, so I try to post around lunchtime. Also, it's the only platform where it's acceptable to spam hashtags. So figure out what hashtags are popular and run with it!
And take advantage of trending hashtags and try to stay ahead of the curve. On National No Bra Day I posted an "awkward no-bra hug" animation on Instagram with the hashtag #nobraday and it performed really well compared to my other posts.
Part 2 of your post really resonated with me. I'm one of those people who is embarrassed/anxious abut showing off my comics to friends and family. So I started at like 2 followers on each of these platforms and built up from there. I could easily get my content exposed to 300+ people on Facebook, but I'm so caught up in "But what if [insert super religious family member here] sees my comic about [insert potentially offensive subject here]??" that I haven't started a fan page, and may never start one. :c I thought I was the only one with that problem.