I truly believe Foursquare has done a fabulous job with this, as they have made a game out of location based sharing. I believe the fun factor is what's missing from most location based software, as people want to know where you are, but they don't want to share at times (ie. when you are at home, you don't announce you're home, etc.). If you make any location based social application a fun game, people will definitely be more inclined to share their location. This has been proven with Foursquare.

Another thing that I believe is missing is the ability to aggregate all these applications together under one umbrella and keep track of them all, so maybe there is some kind of directory or umbrella site needed for people to keep track of their location and share it.

So, if you use Foursquare, Loopt, Google Latitude, Gowalla, GyPSii, Fondoo, Brightkite, Fire Eagle, Stalqer, AmigoMapper, GetMyWingman, Tooio, GPSME,Flaik, Locaccino, EagleTweet, Toai, Nulaz, Mizoon, Snikkr,Match2Blue, MyAdventures, Glympse, Geo-me, Shizzow, Buddyway, Buzzd, Locatik, Buddy Beacon, Socialight, ComeTogethr, Centrl, SpotJots, Jentro, Mobilaris, TownKings.de, CitySense, Zintin, MyGeoDiary, Twittelator, WeNear, SpotAdventures, TripIt, Dopplr, PocketLife, iCloseBy, Qlique, MapMe, Fraced, FindByClick, MyQiro.de, Skobbler.de, Nowhere.de, WhereYouGonnaBe, MeetNowLive, Locr, Trapster, iPling, WhereIs.com/everyone, Trackut, Bliin, Twinkle, etc. at the same time, wouldn't it be neat if there was one service where you can update them all at once, and simply use just one service? The reasoning behind this is that some people like to use one, a few, or them all, but seldom do they update them all at one time! There is a huge list of companies in this space, so I don't know why there hasn't been one aggregator that works with multiple services.

There is a huge list here, scroll down to each month and you will see this list.
http://bdnooz.com/lbsn-location-based-social-networking-links/

As for big ideas, I have some other big ideas, but I don't feel comfortable sharing them all :)

I agree with @easyeboy. I'm a long time user now of BrightKite and I have tried to keep it up to date, but that is difficult to do and the interest to update depletes. Adding the game element is a really good way to keep people coming back to update there location. I like Foursquare, but due to the limited locations it can be played I have started with Gowalla. This application has the game element and is playable around the globe.

I think the problem with location based social applications is support for smaller cities/towns. Foursquare is awesome, but if you live in Taber, Alberta, Canada, it's not much fun! I know it's hard but support for smaller towns/cities might help exploit the long tail of users.