Has anyone made a app or successfully auto posted a blog post to facebook and twitter ?
There are quite a few services offering RSS feed to Twitter / Facebook, this is the approach I would probably take. Zapier is one of these:
So you could just make an RSS feed for the blog as normal and then use this to push to Twitter / Facebook.
Services like Zapier look expensive e.g. nearly 500 dollars after two years. Great if you have money to burn.
Perch CMS lacks an affordable one off cost quality add-on for this like other CMSs e.g. https://www.devdemon.com/expressionengine-addons/campaigns
I don't see what would stop anyone writing an add-on like that for Perch.