What is Facebook going to announce at its big press event on Thursday?

In case you haven’t heard, Facebook is planning something. Again. The last few times the social network has sent out invites to the media, we ended up with Graph Search, a redesigned News Feed and the launcher-slash-app that is Facebook Home. So what can we expect to be announced at the mysterious event to be held tomorrow?

The rumours are flying. First, those sneaky unnamed sources and a few lines of hidden code raised suggestions that Facebook was working on an RSS reader to capitalise on the millions of Google Reader users who will be left without their daily fix when the service shuts on 1 July. This could be coupled with the social network site’s renewed focus on fostering public conversations — when Facebook product manager Greg Lindley announced the availability of hashtags last week, he said that the team would be continuing to roll out a number of new features in the future, designed to “surface some of the interesting discussions people are having about public events, people, and topics.”

Could Facebook launch changes to make it even more of a news source and discussion platform? Maybe, but perhaps not in time for this event — some reports have since backtracked on the Facebook Reader rumour in favour of another one: Instagram video.

Perhaps hoping to offer its users an option comparable to Twitter’s Vine app, the latest word out of Silicon Valley is that the 20 June event will be the first Instagram-only product launch event since Facebook acquired the startup last year. While Instagram has more users than Vine, the app’s six second clips are already more shared on Twitter than Instagram shots. Instagram’s video feature is rumoured to allow users to film and share 5-10 second long clips with their followers.

See Facebook, this is what happens when you send out a cryptic invite bearing a coffee stain and the words “A small team has been working on a big idea. Join us for coffee and learn about a new product.”

But there is another possibility which fits with the invite — Wired suggests that the announcement may be related to Facebook WiFi, a service Facebook has been trialling at its Menlo Park headquarters and surrounding coffee shops. Initially devised during one of the company’s hackathons, the service gives users free WiFi if they check in using Facebook. Facebook gets some location data, you get to connect to the web. It’s also a rumour which fits with the tone of the invite — according to reports, small team of three was assigned to test the system, and Cisco has recently announced that it’s including Facebook log in capabilities of some of its wireless routers.

Whatever the new product turns out to be, we haven’t got long to wait before Zuckerberg lets us in on the secret.