When I open my text and attempt to reply I can not do so by using my keyboard. It will let me type one word and lock up. It then goes takes me back to all my text messages. If I use the keyboard on screen I have to continually reopen the keyboard. It stalls when typing words and it extremely incovenient!
I apologize for the delayed response as I understand how important having a working keyboard is in today's day and age! Does this only happen in the messaging app and Facebook? Have you tested other apps like composing an email in Gmail? It is possible there is some corrupted data in the phone related to the keyboard. I would suggest clearing the keyboards cache and/or data within the phone.
Choose Manage Applications
Select Multi-Touch Keyboard
Clear cache and/or data
Please let me know if I can troubleshoot further to find a resolution for your keyboard concerns.