Seeing the explosion in growth of Twitter right now, it’s pretty clear that the hot trend on the web is to have a service which acts as a central hub for information, and allows third party sites and services to be built on top of it. For most of its life, Facebook has been almost the exact opposite, insisting that developers work from within its walls to keep much of the data — and the users — there. Tomorrow, it looks like Facebook may be knocking down its dam to let its streams of data flow more freely.

At an event set for 4PM PST tomorrow, Facebook is expected to announce that third-parties will now have access to data from the site that was previously unavailable. The Wall Street Journal has a few of the details, including that developers should be able to access the all-important photos and videos that users upload. Apparently, these third party developers, assuming they get users’ permission to use this data, could build their own sites and services with some of it.

Also of note is that apparently Facebook will begin supporting more technologies that rely on open standards for transporting data. It’s not yet clear exactly what this will mean, but presumably it could help alleviate some of the issues TechCrunch wrote about last week in noting that Facebook, Google, MySpace and others were creating what were essentially proprietary profiles, that forced all of us to actively use and update all of their services.

We’ll keep you posted on the event tomorrow and will reserve further judgement on just how big of an impact this will have until then. If Facebook really is opening most of its data streams, it would seem to me it’s a smart move to stop some of the momentum that smaller rivals, like Twitter, are getting. After all, Facebook still has its big stick — over 250 million users and more importantly, their data — now it may be able to swing it.