It finally happened. After watching the press go agog over the millions of users and millions more check-ins through popular start-ups Foursquare, Gowalla, Gypsii, Brightkite, and others, a heavy hitter is getting into the game with their own local service.

Facebook - once a rumored paramour of Foursquare - the popular social network will in fact launch its own service. How it will work is still shrouded in mystery but its decision will have wide ranging ramifications for all start-ups in this space while ushering in the notion of mobile social networks to millions of more users.

Why is Facebook a threat?
400 million active users
500 billion minutes per month spent on Facebook
100 million active mobile users - nearly 100x more than Foursquare

Some companies may be safe while others are immediately put at risk:

Independent Mobile Social Networks. Companies like Foursquare, Gowalla, Brightkite, and others without any white label solution are in the most immediate risk. Current users are not likely to abandon their services of choice but winning over new users will become a challenge. Will Facebook users really want to create an entirely new social network? Probably not. The result will be slower growth and less hype. The result is a loss of mindshare and the innovative partnerships that come with it.

White label solutions. White label solutions are probably still safe since Facebook is unlikely to share revenues with carriers who want to collect a percentage of local mobile advertising dollars generated from mobile social networks. Therefore, companies such as Gypsii, who have partnered with China Unicom and Telefonica in Latin America are safe - for now. However, if users opt to not use these services instead choosing Facebook even this more sound business model will be at risk. And with Facebook working with 200 carrier partners globally this could be a very real scenario.

Games. Booyah's MyTown has grown to more than 2M users since December with a majority of users logging more than an hour of gameplay per day. The Monopoly-esque location based game is unlikely to be impacted by Facebook's decision and may be the forbearer of the next generation of location based social networks to crop up after the check-in phenomenon.

It would be irresponsible to say that Facebook's location play will quickly kill the location based market. However, Facebook cannot be ignored. The big mobile social networks may now have to start thinking about a short term exit strategy. Despite fears, Facebook can trip along the way giving hope to today’s players:

1. Implementation. Google Latitude has failed to catch fire and arguably Google has many more users than Facebook - so a failure to properly implement could de-rail the service.
2. Interaction with the rest of the service. Users are already frustrated with all the changes Facebook has made - will adding check-ins - to an already crowded news feed infuriate them further?
3. Partnerships. Facebook will need to create compelling partnerships with small business to large brands in order to build a buzz around the new product and get users excited about using it.
4. Privacy. Facebook will have to tread lightly around privacy concerns in regards to sharing location information. A failure to protect users will result in the mass market being turned off.

In the end however, Facebook's decision to launch a location product may be bad for competitors (despite what I’m sure are pending quips that a rising tide raises all ships) but it is good for mobile social networking, good for small business hoping to partner with a more powerful local advertiser, and good for carriers hoping to educate users on mobile social networking.