Griff, I highly suggest checking out the hiking groups at Meetup.com as a starting point. There are a variety of groups depending on the level of hike you’re after, and it’s a great way to meet other active hikers.
You might also check out #hikerchat on Twitter each Friday at 9am Pacific. There are hikers from all over the world participating, and quite a few from SoCal.
I’m sure you’ll have no problem finding companions for your SoCal hiking adventures.