How can I contact my friends that have no Apple products (iOS or OS X) in similar manner to using FaceTime or iMessage?
Well, you cannot use iMessage or FaceTime with non-Apple devices. With iMessage on the iPhone, the phone will switch to SMS messages when contacting non-iPhone users, but that's it.
Fortunately, there are alternatives to FaceTime and iMessage that are cross-platform.
Alternatives to FaceTime:
Alternatives to iMessage:
I recommend Telegram. It's fast, well designed, works on every platform (including web browser), requires no registering ala whatsapp, and provides sophisticated encryption options if privacy is a concern for you.
Using it for years with both iPhone and Android friends without any issues!
Check up this answer here. I love Talkatone because it uses Google Voice -- unfortunately Google Voice supports only US to provide a Google Voice number (this may change in the future). Anyway, the Google Voice -app is a god-sent for cheap international calls and sync with with Google Contacts and Facebook contacts.
Instead of hacking with apps such as Skype, I chose Google Voice and Facetime/iMessages although it costs a bit to use but better integration and no interest to set up new accounts to new services.
For an alternative to iMessage, check out Viber — it works on all popular mobile platforms and integrates with their address books.
WhatsApp has matured of late and has seen wide adoption across various mobile platforms. They even have apps for Windows PC and Mac along with mobile apps for major platforms and a Web app.
WhatsApp has added Audio and Video call features (which are quite similar to FaceTime Audio and Video), and the recent version are integrated deeply in iOS and Android. Audio and Video calls can be received just like regular calls, even when the app is not running (but an active Internet connection is required).
It is mandatory to run WhatsApp using a mobile app, as the new account registration is done using an active mobile phone number. The WhatsApp mobile apps needs to be running for Desktop and Web app to work.