Facebook has sent out an email to US journalists inviting them to an exclusive event to be held on Monday 15 November in San Francisco, as part of the Web 2.0 Summit.

It pre-empts Mark Zuckerberg's conversation at the Summit the next day.

However, it's not so much the words on the invitation that has got people excited over the potential subject matter for the gathering, it's the invite itself. With the framing artwork made to look like an envelope from the American postal service, combined with the icon for Facebook's current messaging service, the current thinking is that the site may be about to launch its much-rumoured email client: "Project Titan".

The Unofficial Facebook Resource, All Facebook, suggests that "the invitations show two icons of inboxes - the same icons Facebook uses in [its] mobile apps, which spark the theory Mark Zuckerberg may be discussing Project Titan".

We talked about Project Titan on Pocket-lint back in February, so it's not exactly a new rumoured concept, but this is the first time that Facebook itself has teased something that seems to fit. It'll be interesting to find out, on Monday, whether we were right that it will be a rival to Gmail and "a fully-featured email product that'll offer full POP/IMAP support and an address that matches your Facebook vanity URL - if you have one".

Certainly, such a massive move for Facebook would comply with what Henri Moissinac, head of mobile, told us in a briefing last week after the launch of Deals, Places and Groups: “We had to remove things from the announcement because we thought that it would be too cluttered", he said. "A lot of things that we wanted to talk about, we decided that it wasn’t the right time. Watch this space".

So, we're watching.

Of course, it could be something else entirely. Either way, we'd be amazed if it didn't offer significant improvements over the way messaging works on the social networking site, at the very least.

Facebook email? Or do you think we're barking up the wrong tree? If so, what do you think that Facebook may be about to announce? Let us know in the comments below...