Sims crave social interaction, whether it be externally or internally. Which a Sim prefers depends on his or her personality. An introverted Sim will replenish this motive more efficiently chatting metaphysically online with other Sims, texting, or daydreaming about other Sims in their heads. Extroverted Sims will improve the motive in these methods, but would either find the interaction unsatisfying or drop it in favor of calling another Sim on the phone and entering a phone call or inviting the Sim over. Parties are favorable to extroverted Sims, as multiple social partners will fill the need completely in little hours. These often are a fear for introverted Sims who don't have the Popularity aspiration and/or being too young to be comfortable in a crowd.
Social is easy to raise on a community lot if the Sim dislikes parties or is too busy to plan one. Depending on game performance, you can see two or three Sims around or most of the neighborhood. In The Sims 2, you can force this number by entering a cheat. Sims can also take others on vacation or go on an outing. Despite first assumptions, all Sims only need one other social partner to have this motive fulfilled.
In The Sims 2, if the Social bar is in the red for too long, a Sim will hallucinate, which is represented by the appearance of the Social Bunny, an adult-sized NPC bunny that can only be seen by the Sim that summoned it. Early in the development of The Sims 2, social was represented by two motives; friends and family.
In The Sims 3, going to work will increase a Sim's social bar over time (because they are with their colleagues). This is especially useful if the Sim does not have time to talk to friends. The bar won't shoot right up to the top, however by the time the Sim goes home the bar should be decently filled.