Acquia, a Burlington-based provider of services for users of open-source Drupal web content management, on Tuesday announced that it's grown its ranks with the acquisition of spam-blocking company Mollom. Like Acquia, Mollom was co-founded by Dries Buytaert (pictured) who continues to be Acquia's CTO. Company CEO Tom Erickson told me earlier this year that Acquia has strong prospects for an IPO when the timing is right. On Wednesday, the new Inc. 500 list showed Acquia as ranking No. 8 among the country's fastest-growing companies, with $21.8 million in revenue last year.