I think the energy acquisition metaphor is needless and misleading. Extroverts do tire of stimuli eventually. Besides, it's a continuum. Those near the introverted extreme of the spectrum find themselves overstimulated and exhausted from relatively little external stimulation whereas those near the extroverted extreme have a very high threshold for overstimulation. Instead of overstimulation, very extroverted people easily suffer from understimulation, also known as boredom. It should be noted that social stimuli are not the only kind of stimuli to people demonstrate a preference of extroversion vs. introversion. It is possible to be satisfied with relatively little social stimulation but require a lot of sensory stimulation, for example. There is actually a wide range of external stimuli an extroverted person may use to keep boredom at bay.