It is dependent on where most of your contact are. If alot of your contacts are on Facebook or Twitter then use those social networks. If it's WhatsApp then go for it and download the application.
However, if you want to do more than message people (that's send pure text messages) at a cheap and affordable price (some say it is almost free!), then use biNu. It not only allows you to use very little data (alot like Opera Mini) but it also store all your favourite apps in one place, thus turning your feature phone into a smart phone.