Compatible with iOS4 (dropping sometime today), Microsoft's Windows Live Messenger app for iPhone, iPad and iPod Touch combines IM functionality with Facebook, MySpace, YouTube and Flickr support. It's a free download, and even has Hotmail support built-in too.
While Twitter may be the noticeable missing gap in all the social-networking features, it doesn't sound like a bad little app for anyone who still lives in the glory-days of the late '90s/early '00s, when MSN IM was the sole reason I never handed my homework in on time. It also works with Y! Messenger, so you can chat to both sets of friends within the one app. Notifications for new IM messages received are a standard, and you can share photos from your phone in conversations with friends, or upload to albums. Out now, it's a free download. [iTunes via Redmond Pie]