There are multiple open source tools that can manage social media accounts for you. Here are two:
Socioboard offers an open source version and a commercial version. It supports Facebook, Twitter, LinkedIn and Instagram (but not Google Plus). From the website:
Socioboard Core is an open source social media management, analytics and reporting platform. It supports nine social media networks. Using Socioboard Core you can monitor and manage multiple social media accounts and create useful reports and take business decisions based on them. Socioboard Core is available as a web app and mobile apps on iOS and Android. Its completely open source and fully customizable and extensible in the form of plugins. It has a commercial SAAS version at – www.socioboard.com and open source version at – www.socioboard.org
CampaignChain is free, open source software that can help manage accounts for Facebook, Twitter, Google Analytics, LinkedIn, and more.
In addition, below are links to two blog posts that list several alternatives. The first link also includes a chart comparing some of the programs.
Disclaimer: I have not used or tested any of these apps.