Facebook sent out press invitations via snail mail for a launch event, possibly for an RSS Feed or news reader, at its headquarters in Menlo Park, California on June 20.

The Facebook launch event invitation postcards request that reporters come and see the "big idea" that their "small team" has been working on, Mashable's Emily Price reported.

 "A small team has been working on a big idea," the coffee-stained invitation reads. "Join us for coffee and learn about a new product."

Facebook's previous launch event at headquarters announced Facebook Home, which hasn't been very well received.

In 2013, Facebook has thus far announced the launch of Facebook Graph Search, and its updated timeline design.

Tech reporters are guessing what Facebook might have in store for its announcement, as the Mark Zuckerberg-helmed company just this week revealed that hashtags, made popular by social networking site Twitter, would be added as a new feature.

One possibility is a Facebook postcard service, which Facebook just tested recently. The service allowed users to send physical copies of Facebook pictures through snail mail.

Facebook allegedly told Mashable in November that it was testing price points for the postcard service before public launch. The snail-mail invites suggest that the post card service is a possibility.

Other rumors suggest that Facebook will launch RSS feeds, which would allow Facebook users to subscribe to online content from news organizations, blogs and videos.

Facebook announced earlier this week that it would soon announce new public conversation features, according to Joanna Stern of ABC News, and the RSS feed would definitely qualify.

"Hashtags are just the first step to help people more easily discover what others are saying about a specific topic and participate in public conversations," Facebook's Greg Lindley wrote on the Facebook blog. "We'll continue to roll out more features in the coming weeks and months, including trending hashtags and deeper insights, that help people discover more of the world's conversations."

Given that Google Reader has recently been killed, the introduction of a Facebook-supported news reader seems likely.

TechCrunch's Josh Constine agrees that a Facebook RSS feed will probably be launched at the event.

"A news reader app and web product could take advantage of Facebook's massive treasure trove of aggregate data on what people share to surface popular and personally recommended news articles," Constine wrote.

But why the coffee stain on the invitation?

Constine noted that coffee stains are frequently found on newspapers, when people read them in the mornings over coffee and breakfast.

The event on Thursday, June 20 will shed more light on Facebook's latest innovation.