Sims crave other Sims, especially if they are Outgoing. Sharing a meal with a family member, hopping into the hot tub with some friends, or catching up with a neighbor are some simple ways of making sure the Social meter doesn't drop into the red. Interacting with other Sims usually boosts the bar, but sometimes it doesn't. For example, if Sims have a negative conversation, or come on too strong (flirting where it isn't welcome), their Social rating will drop. Also, fighting or arguing with a rival won't do the Social meter any favors either.
Social is also easy to boost in areas such as Old Town and Studio Town. There are plenty of people wandering around to talk to if you need to. Your family goes with you to Vacation Island, so there are plenty of social opportunities there. As for Downtown, if you're taking a date along with you, your Social bar shouldn't really be emptying.
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.