Recently, my Samsung Galaxy S i9000 will not let me go on FB, Twitter, or anything that requires Internet. It will also not let me download anything from the Market.
Check the internet connectivity in status bar (top bar), if alphabet 'E' is displayed, then internet connectivity is fine otherwise contact your service provider for activating EDGE/GPRS services.
If alphabet 'E' is displayed and internet doesn't work, then turn off your mobile, take out the SIM and reinsert it after 10-12 seconds. Turn on the mobile and check. Now internet connectivity should be restored.