Because, by default facebook use bangla unicode font for showing bangla texts.
That unicode font is so small and complex to read bangla texts.
You can easily solve this problem by only just few steps, and you will then see more clear and bigger bangla font in facebook or twitter etc....
Steps : (For FireFox Browser)
- Go to Tools option from top menu bar
- Click on Options
- Click Content tab, then select 'Advanced..' from Font & Color part
- Select Font For ----> Bengali
- Proportional ---> Sherif (Size : 26)
- Select any bangla unicode font (eg. SolaimanLipi, or SiyamRupali) for Sherif box
- Set Minimum Font Size : 26 or 20 and Click on OK
Steps at a glance :
Tools-->Options-->Content (Tab)-->'Advanced..'-->Font For : Bengali -->
Proportional : Sherif (Size : 26)--> Sherif : SolaimanLipi -->Set Minimum Font Size : 26 --> OK