Everybody is free to have their own opinion on whether to join some social network or not. However, Linux Mint is very much on Facebook. Like on many other social networks (we're at least represented also on Twitter, Tumblr, and reddit), users themselves create communities for common interests. So also for Linux Mint
Some are bascially a mirror of Planet Linux Mint (which combines news from the blog, segfault, and mintcast), while others are from Linux Mint local communities (so not in English), or even like on reddit another way to get support for Linux Mint or just talk about Linux Mint.
Side note to those inspired to add Linux Mint on their favorite social networks: Linux Mint is a trademark owned by Clement Lefebvre so it is best to get his permission before using the Linux Mint name or identity.