The Electronic Frontier Foundation has posted a list of secure  alternatives to various operating systems, web browsers, email/IM clients, and so on at prism-break.org.
I've used Linux as my desktop for a while, from around 2000-2005. I am sure that the user experience for Linux on the desktop has improved since then. I am most familiar with Fedora-based distributions, but what little experience I've had recently with Ubuntu has been excellent as well.
Firefox has started off as a decent browser alternative to IE/Safari, and has continued to improve as the years have gone by. The only reason I continue to use Safari at this point is inertia.
I have used Evolution, KMail, and Thunderbird at one time or another, though not with PGP support. All are quite serviceable email clients. Of the three, as far as I know, the first two are Linux-only, which leaves Thunderbird or a derivative if you're not interested in jettisoning your OS.
Pidgin and Adium are both good chat clients. Adium is my current client of choice, and again, has multi-platform support.
Social media, though... hmm. Moving to a different social media platform (from Google+, Facebook, LinkedIn, Twitter) has an inherent problem: the value of a social network is not in the software, but in the number of people who are using it. For most people, dropping Facebook for Disaspora would be like switching to an alternative phone network that can only connect to other people who also switched to the new network. Sure, there's a tipping point at which it might be useful, but you're working against a huge amount of inertia in trying to get people to switch. Just look at the trouble Google+ has had in building a social network that competes with Facebook.
Overall, replacing insecure software with secure (well, again, more secure) software is certainly possible. Is it probably? Unlikely, unless you're really, really motivated.
Putting together a personal security minded Linux distribution (or sets of packages for various distributions) would be a definite win, though. It's far easier to decide to install one package that includes a bunch of related tools, and give them a try, than it is to dig through four score packages and figure out how to make them all work together. That's especially true for Windows and Mac OS X users who might be interested in using security minded software, but who are not willing to completely abandon their current OS for some reason. 
Overall, an A+ for effort to the EFF for coming up with this list.
 Well, more secure than the current default of letting your electronic thang hang out.
World of Warcraft