I use Rowi for twitter, I used to be an iOS user and I loved tweetbot. I love how fast Rowi, is, and it supports tweetmarker for saving timeline place across clients and devices. They also have a WP8 update in the pipeline but the 7.x version works great. Fastest twitter client I've used. Also you can jump to top easily which is important to me in a twitter client.
For Facebook the built in people hub is quite usable for light use, especially because it is fast and doesn't require launching an app. For more specific things like commenting on a group page I use the Facebook App. Going to touch.facebook.com in IE works pretty well too, I have that bookmarked