The official fb and twitter apps used to really suck tbh, but in recent updates both have gotten really good. Facebook still has a couple minor issues but sees a lot of updates, so these things might even be worked out by the time you decide on a WP8 device. Twitter is a really polished and fairly feature-filled app, but imo a better experience can still be found on third party apps like Gleek!, Mehdoh and Rowi. I haven't upgraded to Windows 8 yet so I can't really comment on the file sharing issue...
I really like the flick concept in the bb10 keyboard. So much, in fact, that I'd like MS to buy BB just to bring this feature.
At any rate, coming from your device background WP has a different philosophy and execution of the smartphone in general. The only way to know if you like it is to try it. To that end, if your carrier has a two week or longer trial/return policy (mine unfortunately does not) I recommend using it to check things out and make the WP your daily driver for that period. Alternately, consider picking up an inexpensive device like the Nokia Lumia 520, which can be found unlocked for around $150, just to learn the software. Bear in mind, of course, that the experience will be faster, smoother, etc on a high end device. After using Windows Phone since 2011 with my HTC HD7 (hated the phone, LOVED the software) I can honestly say I could never use Android, BB, IOS etc. WP is for me for life. Is it perfect? GOD NO! But for my money it's the best mobile OS available.