Yesterday I came across this tool(Your Engagement Bar ) which you can use to keep your members in your community even when they visit external links from your community. In this was you are sure members will return to your site.
Check it out. The instructions seems to be very simple. But if course contact me if it becomes complicated. This is the link http://visibli.com
Ooooh I went for the Free version in case yOu also want to save some money go for it.