Liǎng rén sùwèimóumiàn, què zài wǎngshàng liáo le zhěngzhěng sān nián.
The two had never met in person, yet they had chatted online for a full three years.
Tips
usage
Classic way to describe pen pals, online friends, or someone you know only by reputation. Pairs naturally with 却 or 但 to contrast with closeness: '素未谋面, 却...'
grammar
素 here is an adverb meaning 'always, from the start'; 未 is a literary negative ('have not yet'). 谋面 = 'arrange to meet face to face.'