Every day you will get 5 bonus energy to socialize with each of your neighbors. After you use those up, you should get 1 energy added to your energy bar. If you don't get it right away, go back and visit the same neighbor and it should give you the +1 energy.

If you are completely out of energy, and need to fulfill your social need (which costs energy), go to Bella Goth's house and just stand there. Eventually your sim will automatically talk to her without costing you energy.