Twitter and Facebook both are very popular in India and starting its Hindi service will surely give a boost to twitter in India. Facebook already supports various Indian languages.
India has been important to twitter as in last year Twitter Local trends included India and Mumbai . Also it has an official twitter India account @twi (though @twi hasn’t tweeted yet about the twitter Hindi Service being launched.)
(Source: One hundred million voices )
Though if you want to tweet in Hindi or other Indian languages, you can do it now - Tweet, chat, do Presentations in Hindi and other Indian languages with Microsoft Indic Language Input Tool