Customer support startup Zendesk has raised $19 million in Series C funding led by Matrix Partners with existing investors Benchmark Capital and Charles River Ventures participating in the round. This brings Zendesk&'s total funding to over $25.5 million.

Launched in 2008, Zendesk offers a web-based, SaaS-delivered help desk / support ticketing application that gives companies a simple way to manage incoming support requests from end customers.

Over the past two years, the startup has managed to gain an impressive client list, including Groupon, Twitter, Yammer, Sony Music, TriptIt, Lonely Planet, Foursquare and MSNBC. And Zendesk is adding around 20 new customers per day, recently hitting the milestone of 5,000 businesses using the customer service platform.

Most recently, Zendesk added a deep integration with Twitter, allowing users to turn a Tweet into a Zendesk ticket. You can also respond publicly to a complaint on Twitter from Zendesk‚a4‚¬‚a4„s platform and record any Twitter conversation that is taking place within Zendesk‚a4‚¬‚a4„s platform.

The startup is also announcing a tighter integration with Salesforce today, which is interesting because Salesforce‚a4‚¬‚a4„s Service Cloud also has much of the functionality that Zendesk offers.

Zendesk CEO Mikkel Svane says that we can stay tuned for a Facebook integration with the customer service platform. He adds that mobile will also be huge for Zendesk in 2011. The company already offers native apps for the iPhone, Android, BlackBerry, and has seen more than 50,000 downloads of its iPhone app..

The startup&'s price ranges from $9 to $59 per support agent per month, after a one-month free trial. The new funding will be used for sales and marketing efforts.