It is rare that I use social networks, but usually I do not decline invitations to connect when they come from people I know. This said, I had doubts when accepting invitations from other Debian Developers, most of whom I never met in person: how can I be sure that the account really belongs to the person ? Luckily, we have a tool to solve that problem, our GPG key ring. I have signed the URLs of my pages on some networks, and from now I decided to only accept invitations to connect that are certified by this way or another, when they originate from somebody having a key.
Paradoxically, although most social networks do not redistribute the source code of their systems, the only time I really used one was for Free software, by contacting a former developer whose email address was not available anymore, in order to clarify the license of one piece of code he wrote long time ago, that way saving one of my packages from being exiled in the non-free area.