Does Google penalize for including the same sentence at the end of every post example, "please follow us at Facebook, and Twitter @" to encourage the readers for following. Is this count as duplicate content?
Of course not! This cannot be penalized as duplicate content. You are attempting to promote a page to Social media like Facebook, Twitter, Google+, etc,
This just shows you want it to spread and reach to most of the people.
Additionally, it cannot be called copied since a few percentage of text is being used on many of your website. That cannot be counted as SPAM content or copied content.
e.g. All the WordPress blogs have "sample-page", and default post with similar text that however changed later but never can be counted as SPAM or duplicate content at all.
In my own opinion and experience @Zirkon Kalti, I don't think it can be counted as duplicate at the end of post, because you are promoting yourself into social media as what @Manish Mishra said. I tried that when my affiliate marketing blog with product reviews before, and I use the same message to each of the end of my blog posts, promoting my own social media accounts. No problems on my part, and I was still ranking then. But of course, Google algorithm can change anytime soon, so who knows they might change the rules of the game again.
Your web page is not penalized for using "please follow us at Facebook, and Twitter @" twice in a post. But i think you can use social button for facebook, twitter and other social sites. You can also use facebook page like button in your page for making more engagement with your visitors.
There is no way to get penalize for such things as every site built in such method even this forum has follow us, share on twitter. Not only those social share text, but we see about author info in every blog post. So no need to worry about these things as search engines have latest methods to crawl.
There are no any rule announced by google for Please follow us on Facebook or follow us on twitter or any social media links.
But for posting social link in your post you can also use social media button like Google+, Pinterest, Facebook page or any one social media button.
And it is best way than using Social links.