Do you want to find like minded collectors to buy, sell and trade with online, without having to pay the significant fees of a broader marketplace like Ebay? Social media can be a great way to connect with collectors, both in your general geographic area, and worldwide.
The best part about Facebook is you can join quite specific groups to focus your collecting. If you follow a certain team or player, there is likely a group for either (assuming the player is popular enough). If you only collect vintage cards, there are groups for that too. Be sure to get vouches (from someone you trust) for anyone you plan on dealing with online, and/or use some of the protections offered with PayPal. For the small fee they charge, it’s worth the peace of mind.
Instagram seems like more of a way to show off your collection, than buying and selling, but transactions definitely take place. You’ll need a rather large following for people to see your cards, as there isn’t the same group feature that Facebook has.