Alright, so this has been on my mind awhile. I think I may have found the key to really getting personal and site-wide plotting going well.
It's very simple.
Make connections. What? Well, there are a few ways to do this.
Link Your Threads.
If you are in a thread with someone, allude to the thread you just finished up. Don't make every thread so disconnected, but try to have them all flow together, so it's almost like every thread happens because of the one before it.
Example. My character Bob goes to the store to buy milk and runs into an old flame named Tess, and he ends up buying cake mix just to make sure he ends up in the same aisle as her. Next thread - his best friend Larry is coming over, so I add in a part about him trying to bake a cake, but it catches the kitchen on fire. He now has to explain to Larry about the cake, and Tess as well. Now Larry has a connection to Tess, even though the characters have never interacted.
Spread Information
This works best, I think, for students and Ministry workers. However, it works for everyone. Because this is GOSSIP. If a character tells your character something, it is now your information to give away. No point in confiding a secret if there is no risk of it leaking.
If your character have an awkward moment with someone, have them casually mention to their sibling "Man, that Bob is such an awkward guy." Now that sibling has some context/bias when they meet Bob.
Or perhaps they say "Y'know, that Bob guy sure was acting weird. It's like he's up to something bad." And your character knows Bob's girlfriend, so now you tell them, and it gets back to Bob.
Once a character gives you information, don't just store it away to never be seen again. Let it influence your character, let it influence others.