As nieldk it seems like this fix was first made for N9 and after he patched it for N900 also. As nieldk says in his post that problem was in telepathy-gabble library. After i installed the fix now Facebook IM is now working without any problem and showing all contacts. Here is a screenshot after fix is installed.
Here is how to install this fix:
First download .deb file from HERE and place it in MyDocs or N900 if you are copying it from PC (don't paste it in any folder).
after that Open terminal and type:
cd /home/user/MyDocs/ (enter)
dpkg -i telepathy-gabble_0.8.13-0maemo3+0m5_armel.deb (enter)
When done installing give N900 a restart and then check your Facebook account. Everything should be working now. All credit goes to nieldk for his awesome work.
Link for original thread at TMO: Click Here