Hello PiscesPassion:
The short and simple answer is that you cannot print "decent" prints from flickr and (especially) your Facebook photos, for the simple reason that you need original scans of high resolution to make poster size prints.
Even if you started out with a high resolution digital image (of let's say 300 dpi, which you need to make decent poster-sized enlargements); Facebook's uploading process will automatically reduce the size and resolution. Larger resolutions take up more space, which costs the hosting sites more to store them.
To make a decent 4x6 photo, you need an original image with a resolution of at least 100-150 dpi, and with a minimum native size of 4x6. An uploaded picture with a native size of 4x6 at 72 dpi, which will produce a 4x6 picture on screen, will produce a soft "pixelated" photo print. Anything larger (5x7 or 8x10) would be even more severely pixelated to the point of being unrecognizable.
Elaborating what Steve P said, these photosharing sites are not suited for storing your photos for printing enlargements from. In fact, you shouldn't put any photos on those sites that you wouldn't mind "losing forever"--if the site went belly-up.
Any photos that you want to make enlargements from should be stored either on your desktop (computer's hard drive), an external hard drive, USB thumbdrive, or on CD/DVD at resultions of at least 150 dpi (for making 4x6s) and 300 dpi or larger--if you want to make 8x10s, 11x14s or larger poster-sized prints.
Hope this clears things up.