Meetup's website remained offline Monday, the victim of a massive DDoS attack, which began last week with an email asking the company's CEO to cough up $300 -- or else.The social networking site and its mobile apps have been largely inactive since last Thursday, when the distributed denial-of-service attack began on its servers. At least several attacks have been launched at the site, causing many hours of downtime over several days, which the company says is its first such experience since it was founded 12 years ago.