It all depends on the Sims and family in question. I do find I often wait for a walk-by or neighbor to come by so I can greet them and make friends that way...or online works too and calling them to chat. Why a Sim would make friends depends, could be needed for a promotion, could be for a storyline or as simple as they need social.

To maintain their friendships I just usually chat with them on the phone when social is low, generally works. Depending on the family depends on how hard it is but if I care enough it's not generally too hard but sometimes can be challenging, if say it's a large or busy household. I don't always care to keep up with friends either and sometimes they fall by the wayside. It depends on why they became friends in the first place.

Friends brought home from school or work work well too. It all just depends on the Sims to me I guess and what I want to do with them. I am also starting to try and visit community lots more and Sims do what they will in the background.

My process is pretty organic. I do rotations, and send my sims to community lots quite a bit. Everyone works on being social. My popularity and.or outgoing sims know a lot of people. Extended families help create a rich web of contacts. And children! Children are a very good social networking tool (one little trick: if someone calls a child after 7PM, a parent from that household will show up on your doorstep, because the kid is deemed to be temporarily on your lot, and they want to take him or her home!).