Is this a scam or legit job interview through Instant Messenger?

[T-Niecey]

I have an interesting story. I found an ad on craigslist for administrative assistant/office specialist position, the ad directed me to go to the company's website and apply. I researched the company & they have an actual site, contact numbers, address, LinkedIn profile, Twitter, and Facebook. I went ahead and applied and earlier today I received an email that had the hrbenefits@companyssite.com wanting to conduct an preliminary interview over Yahoo messenger. The email have me a time & date and advised if I'm unable to be available email them back with three times I am. The company seems to be local & probably small but an interview through an online messenger seems fishy. They didn't leave their contact info (like whom I would be speaking with). I'm thinking about calling the corporate to determine if it's legit but I don't want to jeporadize the opporrunity. Has anyone ever been in this predicament and turned out it was legit or should I just assume it's a scam?