regular twitter users who "get it" try to convince the other folks by mentioning the various benefits they have experienced - thereby doing the free promo for that tool. fair enough.
the crux is, that for every enthusiastic user you have several who feel unlucky. and the few testimonials obviously don't suffice to get a considerable amount of people motivated to at least try this thing out. the sheer fact, that overall only few people actually ever subscribe to twitter is the first problem.
the second problem lies within the entry barrier after registration. fast satisfying user experiences are hardly achievable with twitter. either intensely devote yourself to the tool and dive deep into the networking thing for months - or simply dabble in it with a few tweets and follows here and there and quickly get bored in most cases. and the latter represents likely even the majority of the 8 percent registrants. a huge number of disappointed users is always deadly. so twitter will hold a niche status if at all.
so, that leads nowhere. it doesn't feel right and there's little reason to be optimistic about a real twitter breakthrough in the future. in fact, as already mentioned, the curve of unique users is already flat for some time now.