Matthias, welcome to the Philadelphia folk scene, as active a community as exists anywhere. My first advise is that you join the Philadelphia Folksong Society. For about $35 a year you will get admission to nine concerts, a slew of house concerts and workshops and you will recieve TUNE UP which includes schedules and locations for folk events and clubs.
A good place to hang out is the Mermaid Inn in Chestnut Hill. They have something going on every night and their schedule is eclectic.
They are located at Germantown Ave. and Mermaid Lane.
I guess I forgot to mention that you can reach the PFS at 215-247-1300