I grew up in San Diego and still have family there, so I still spend a lot of time there (no intention to move back though, too crowded).
San Diego county has lots of sub-regions, and you'll find like in most big cities, people stay mostly in they're own small sub-section. There are a lot of free things to do and lots of Meet-up groups, etc. So whatever your interests, you can find people pretty easily. That said, mostly the out-going and single people are going to be found nearer to the coast, but there are a lot of people in general, and always a lot of people moving in and out of the area, so it's pretty welcoming to new-comers.
Chula Vista can get HOT in summer, there are definite micro-climates, so don't be expecting the weather that is reported for San Diego (recorded at the airport right on the bay).
You didn't say if you're single or moving with family?