I keep getting this come up when i log into facebook.....check6only.facebook.comajax/v6.php?v=check6only&pingonly=false. i can get on the internet ect its just fb.

First, update your java runtime environment software to the latest version for here:
http://www.java.com/en/download/manual.jsp

Then update your Adobe flash player software to the latest version.
For Internet Explorer you need this update:
http://majorgeeks.com/Adobe_Flash_Player_for_Internet_Explorer_d5163.html

For Firefox, Netscape, Opera and Chrome use this update:
http://majorgeeks.com/Adobe_Flash_Player_for_Firefox_Mozilla_Netscape_Opera_d7806.html

If that doesn't fix it you might have some viruses and malware.

Start your PC and press F8 several times until the boot menu comes up. Then select "Safe Mode with Networking". Now get some or ALL of the FREE programs below.

Download, Install and Run Malwarebytes Anti-Malware:
http://majorgeeks.com/download.php?det=5756
When Installing choose the non-pro version.

Download, Unzip, Install and Run TDSSKiller:
http://majorgeeks.com/Kaspersky_TDSSKiller_d6895.html

Download, Install and Run Super Anti-Spyware:
http://majorgeeks.com/SUPERAntiSpyware_d5116.html

If you have an outdated Anti-virus program then Download, Install and Run Avira Anti-Virus:
http://majorgeeks.com/Avira_AntiVir_Personal_-_FREE_Antivirus_d955.html
Make sure to select ALL of the "threat categories" when you install or configure Avira.

You should be in pretty good shape if you run and install all of these programs.