Most social networking Apps can already do push notifications and even the built-in Apps can do it too. The problem with all the Push Notification features built-in to most Apps is that they are either on or off. In other words you can't control when you're notified. I first discovered Boxcar as a solution for getting Push Notifications of Google Voice SMS messages and Voicemails. Although the Google Voice Apps all do Push Notifications now, i don't use the built-in ones. I still continue to use Boxcar because of one major feature – Quiet Time! You can set a time range for when you DON'T want to be disturbed each night. I really don't need to know that someone retweeted one of my tweets in the middle of the night. Sure I could just turn my phone off, but often when I traveling I leave it on for potential emergency phone calls from home.
Boxcar does more
Boxcar also usually does more than the built-in push notifications of these apps. For example, with Google Voice I don't want to actual SMS message displayed on screen. You can turn this off in the native Message App in iOS, but not in any of the Google Voice Apps that I've seen. No problem for Boxcar, just switch it to "Private Message" and Box car will just display a generic message on screen until you go into the App that you got the message in. Also with Twitter for example I use the "bird" sound to know right off the bat that the notification is Twitter related.
Setup is really easy and once it's setup you can turn off the push notifications of the individual apps and just use Boxcar for your source of all notifications. I could easily give this App 5 stars, but I'd like to see a couple of things. First of all I'd like to see the list of Google Voice support apps updated faster. For example, right now I'm using GV Mobile + and the latest version isn't supported yet in Boxcar. Also I guess their servers get busy sometimes and notifications aren't instant during those times. I always get them, but not immediately in all cases. If that's a concern for your most important apps then you might want to stick with the native App's Push Notifications. Otherwise, I LOVE Boxcar!
You can get Boxcar for Free here from the 
There is a $4.99 in-App purchase if you wish to disable the ads.
I have been using Boxcar for over a year now! I love getting my Tweets that are directed towards me to pop up like a text message. Now I don’t miss a thing!!