For as long as I can remember, I've needed glasses and contact lenses. I'm pretty much blind as a bat without them. So, when I discovered how to increase the font size of the sites I frequently visit (i.e. Facebook and just about everywhere else) I did the Happy Dance (several times!). Now this might be a trick known to a lot of people already, but *I* didn't know about it, so I'm happy to share.
To increase the size of fonts for most sites online, press the CTRL key while tapping the "+" (plus sign) key. Do it several times if you must, because every time you tap it you'll increase the font size by one increment. It also works in reverse, so if you want to decrease the font size, hold the CTRL key down while tapping the "-" (minus sign) key. Oh, and if you have a wheel mouse, it also works by pressing the CTRL key while you scroll your mouse down, or up, a notch.
As far as I know, this only works in Windows. As I don't have a Mac I have no idea if this trick will work or not for that, unfortunately.
In the meantime, happy increasing! (Or decreasing, if the case may be!)