Register your catalog with the copyright office as it's only a one time expense (I believe around $50.00) then as you write new material you can just add it to your catalog.
I know you can register single songs and collections of songs but both of those are a one time deal. I've never heard of any sort of "open catalog" where you pay $50 one time and can then "just add....(new material) ... to your catalog" as you write it.
Heck I'd have been doing that if it existed. I did look on the LOC website because that would be really cool and a great deal for any writer. I couldn't find it.
Please correct me if I'm wrong
Also, I've never had anyone other than scammers contact me because of a song on a social media site. All of my contacts and placements that are legit, came from me contacting the library/publisher, and them vetting the quality of my music, then signing it.