Whilst I travel for work, video and VOIP calls, as well as text messaging, become my lifeline back to my family. Though they may not always be 100% reliable, I know that I feel heaps closer to home just having access to this technology.
For this purpose, I use the following apps the most:
Skype - I find that this is the most reliable technology for keeping in touch virtually. I think the bet part of it is that even though the other person may not be online, it is possible to call them by phone or send them a text message using Skype. In fact, it's a decent 'replacement phone' whilst travelling.
In fact, I have even used this to connect to conference calls while I'm on the road, without wifi access and just on 3G (only locally, not overseas/roaming).
Skype is even an internal messaging system that we even find extremely useful in the office.
I've also had the chance to test drive group video calls using Skype, and I thought it was pretty cool.
FaceTime - The ease of use of calling someone using FaceTime is pretty good. If you're primarily using Apple-to-Apple products within wifi, then it's cool.
Of course, the downsides are obvious: No easy way to text message within the app (will need something like iMessage), only works with wifi, and no way of using it as a phone replacement.
iMessage - With the introduction of iOS5, iMessage came in to picture. The main advantage of this messaging app is its ability to share pics and text as a group.
How about you? Other than the usual social networks like Facebook, Twitter, Google Plus, etc, what's your favourite way to keep in touch? Preferably, with video/audio options?