foursquare 260x390 Foursquares iPhone app gets sticky for iOS 4.

Earlier this week Foursquare celebrated their 100 millionth check in and moved into new offices.  Today if you search for updates to apps on your iPhone, you’ll find Foursquare at the top of the list, adding a whole host of new features aimed at keeping you using Foursquare and never leaving the app.

The most notable update is the ability to SMS and email messages to your Foursquare friends without ever having to leave the app.  Simply tap on a friend and an email and/or SMS messaging icon will alert you as to what type of message you can send your friend(s).

SMS messaging from within Foursquare works only if your friends have added their phone number to the profile, so strong arm your friends into compromising their privacy to a greater degree by adding their phone number to their profile.

Email works flawlessly.  I bombarded one of my friends, we will call him/her, “my wife,” with emails from within Foursquare and sure enough, I was able to draft the email to him/her, send the email and return right to Foursquare.

Other interesting features of the updated Foursquare include:

All Foursquare:// urls in other iPhone apps now open, without crashing, when Foursquare is running in background.

New high resolution for retina.

Faster background refresh for iOS 4.

Current position locator for all maps.

Despite having no dedicated iPad app, like the great Gowalla, Foursquare has enhanced their app layout for all platforms even the aforementioned iPad.

Foursquare, already one of the location sharing services leaders, has upped the ante with their latest updates.  All of the updates are aimed at keeping one in the Foursquare app 24/7, whether you are actively using it, or simply keep it running in the background.