Hans J Eysenck's personality trait of extraversion is typically measured by asking about sociability, but extraversion is not primarily about sociability: rather, it is an index of reliance upon external stimulus to maintain arousal.
(Introverts are internally aroused, hence do not 'need' other people so much, and so tend to be solitary.)
Effective external stimulus is usually social, because humans are social animals, but nowadays it can also be provided by reading, computer work, absorption in crafts etc.
Therefore, many autistic/ Asperger's type personalities should properly be classified as extraverted, because they require high levels of external stimulus.
However, because of their autistic tendency, social stimulus is not effective (because it is not interesting enough to them) and instead stimulus may be sought from TV, computer games, factual hobbies etc.
As David Healy has noted, many people with ADHD (attention deficit hyperactivity disorder) or ADD (attention deficit disorder) can be seen as extraverted, in the sense of needing environmental stimulation - and indeed there is much 'comorbidity' or overlap between autistic traits and ADHD/ ADD.
Yet people with ADHD may be relatively uninterested in social interaction or social exchange - and their extraversion may therefore be non-obvious.
Also, by this concept of extraversion, more women than men - indeed most women - are extraverted; because women orientate towards other women (real or a virtual peer group provided by the mass media) more than men (on average), and seek constant social inputs.
This applies even if women are shy and dislike crowds, parties etc.
Yet women and men usually come out as being of equal average extraversion when the extraversion personality rating scales focus on social activity - I think this happens because extraversion scales focus on sociability, and these questions about sociability are missing the essence of extraversion, which is not social.
I regard introversion as a rare trait - much rarer than extraversion. The distribution of extraversion-introversion in the population would not be normally distributed if it were properly measured.
Introversion is neither useful nor adaptive for ancestral hunter-gatherer societies (except perhaps of the occasional shaman) but is probably useful for many types of modern (industrial and post-industrial) society, where solitary and autonomous endeavour is necessary; for science, arts and crafts.
Introversion is also a necessity for effectively creative people - creative geniuses - and modern/ industrial society absolutely depends on the work of such people.
But introversion does not fit into an organizational context, because it is solitary, uninterested by other people, awkward, autonomous etc. Hence modern/ industrial society is killing itself by failing to use the unique and absolutely vital (for modern/ industrial society) contribution of introverts.
But introversion may also be pathological, due to low 'drive' or motivation.
A person may be solitary and self-contained not because they are internally stimulated, but because they 'can't be bothered', because they are un-rewarded by action, anhedonic.
And this may be a consequence of a range of diseases and disorders.
This could be either an innate deficit - perhaps hereditary, perhaps a gene mutation; due to infection or a post-infective state; come from mild/ early Parkinson's disease; or due to malaise from immune activation, catabolism (post operative, with cancer), or from chemotherapy; or be caused by anti-dopaminergic drugs, such as the neuroleptics/ antipsychotics, or SSRI 'antidepressants' (indirectly anti-dopaminergic).
Some cases of dependence on dopamine-boosting agents (such as nicotine, or stimulants) are therefore likely to be primarily examples of self-medication, rather than addiction.
But in general, I think it is extraverts that benefit functionally from stimulants (amphetmine, ritalin, caffeine, nicotine), since this provides them with an internal stimulus, making them more independent of their surroundings and better able to concentrate and focus on 'work'.
(This is the explanation of the apparent paradox - noticed back in the 1930s - that 'hyperactive' people - children and adults - are seemingly 'calmed' by stimulants such as amphetamines. They are not really 'calmed' but are made less distractible.)
And, as described before, pathological introverts may also benefit from stimulants because they need more drive and motivation: the stimulants treat their state of insufficient dopaminergic stimulation.
But natural introverts would probably find that stimulants were dysfunctional, making them somewhat manic, so internally-over-stimulated as to be out-of-touch with their surroundings.
I think it was Eysenck (again) that first noticed the almost opposite effects of stimulants on extraverts and introverts: 'calming' extraverts but making introverts jittery and 'hyper'...