There are a number of possible ways. For instance, on my meta site, I use the advertising plugin to run house ads. (Not social media, per se, but a similar idea.) A more aggressive approach would be to put them in a custom header or a sitewide banner. A bit less in-your face is a globally pinned topic with contact information. Or add them to a customized your FAQ.
It all depends on your goals and the culture of your community. So what is your goal?