I think just about everyone in the local advertising halo is digesting Facebook's announcement that they are jumping into LBS with two feet.  The LBS folks knew this was coming, yet the proposed "co-opetition" still scares the hell out of them.  In fact, I brought up this very point at a panel in May with Josh Williams of Gowalla.  The entrance of a major player with a HUGE audience is a massive gamechanger for the startups that popularized things like check-ins, mayorships, location-driven game mechanics, etc.

If rival tech giants such as Google, Yahoo, AOL, Microsoft, and Apple haven't considered Facebook a competitor yet, they're probably now realizing that, yes indeed, Facebook is perfectly willing and able to compete in the local advertising arena and they're a major, major threat.

Overall this is obviously a really big deal and a seismic shift in the LBS business, in local advertising, and perhaps in social networking in general — although it will take several years to play out.

In May, I shared some thoughts about LBS on Mashable and followed up here on my blog.  I thought I'd spend a few minutes today talking about what I think the latest news means for the sector:

1) LBS startups forced into building an ecosystem like never before — I have written before about how LBS startups have been riding the wave of popularity & innovation but have also to some degree tried to control how third-parties interact with their systems.  For instance, even today only foursquare allows write access to their API.  LBS startups are not by any means dead — in some circumstances they will remain the "consumer front end" for LBS.  But to stay relevant, they will need to be a lot more aggressive fast about building an ecosystem that helps justify themselves long-term.

2) Legitimacy to LBS - LBS startups have done a great job of exciting the early adopter market and showing the world the possibilities of local, mobile services.  But none have yet made it to the mainstream.  Why?  Because it is a HUGE undertaking.  Reaching moms in Kansas is just tough.  Facebook changes all of this significantly because they have such a huge audience.  150 million+ people are on Facebook Mobile and over 500 million experience Facebook on the Web.  Now those are mainstream audiences.

However, I do think that this is a situation where the "rising tide raises all boats."  This will legitimize the existing LBS startup players as well.  In fact, I think in the short term foursquare, Gowalla, MyTown, and others will experience growth.  The bigger question, of course, is the long-term impact for the LBS startups.

3) Facebook, the aggregator — With this move, Facebook aims to be the center of all location-based activity.  The existing players are presently invited to "read" data from the Facebook Places API but will also be allowed to write data to Facebook.  What a great way for Facebook to build a robust database off the backs of the startups that popularized the sector.  I think the golden rule needs to be changed from "he who has the gold rules" to "he who has the users rules."

4) Twitter, the afterthought — Twitter just gets a little more marginalized each day.

5) The Cold War begins in earnest — There is absolutely NO WAY that this move will go unchecked by Google, Apple, Yahoo, AOL, and other major players who are eyeing the location sector.  To some degree, this move really forces relatively quick action by these companies who will need to do something to be relevant.  Otherwise, Facebook will own it.  I suppose you can argue that Facebook is already well on its way — they have the audience, they are about to have a ton of data, and they've delivered an immersive experience to users so far.

We live in interesting times, that's for sure.