Hello, South Side, and welcome to Philadelphia PA, land of hoagies, cheesesteaks and soft pretzels!
Can't help you too much with South Philly or Center-City venues, being from the suburbs and all, but if you can make your way north to Germantown Avenue, I'll tell you THE place to go to meet musicians who can tell you about all the other great city venues: The Mermaid Inn. Check out their website at http://www.themermaidinn.net
Do be sure to check out the map before you go, because the street address is deceiving (Winston Road cuts a diagonal above the intersection of Germantown Avenue and Mermaid Lane). Basically, get to Germantown and Mermaid, go up the hill on Mermaid for a couple of yards to the next intersection, and the bar is on the far right corner.
You may also want to consider joining the Philadelphia Folksong Society -- website is at http://www.pfs.org They have lots of house concerts, get-togethers at people's homes for singing, a monthly concert in a college auditorium in Fort Washington PA, and of course the annual Folk Festival in Schwenksville PA (which is coming up later this month! August 22, 23 and 24, to be exact!).
A smaller, friendly group meets in Bucks County PA if you care to venture that far out of town: The Bucks County Folk Song Society -- website is at http://www.bucksfolk.org
So there ya go; there are some leads to help you hook up with some folks with whom to make music; they (make that "we"!) in turn can help you locate specific venues, record stores, folk-instrument shops, etc.
Have fun, and hope to see you in one or more of these soon!