Perhaps surprisingly for such social endeavors, open source projects typically make only limited use of what most people think of as "social networking" services. But this seeming omission is really a matter of definition: most of the infrastructure that open source projects have been using for decades, since long before "social networking" became a recognized category of software, is actually social networking software even if it isn't called that. The reason open source projects tend not to have much presence <em>as projects</em> on, say, Facebook is just that the services Facebook offers are not well-tuned to what open source projects need. On the other hand, as you might expect, the infrastructure these projects have been using and improving for many years is quite well-tuned to their needs.
Most projects do use Twitter and similar microblog services, because sending out short quips and announcements that can be easily forwarded and replied to is a good way for a project to have conversations with its community; see LibreOffice's "@AskLibreOffice" tweet stream at https://twitter.com/AskLibreOffice for an example of this. Projects also sometimes use services such as https://www.eventbrite.com and https://www.Meetup.com/ to arrange in-person meetings of users and developers.
But beyond lightweight services such as those, most free software projects do not maintain a large presence on mainstream social media platforms (though individual developers sometimes do, of course, and they may discuss the project there). The reward the project gets in exchange for that investment of time and attention appears not to be high enough to be worth the effort.