Here's one answer to the bonus. In short run, people will connect/reconnect people they wouldn't otherwise have--people with whom they share more in common than their current spouses--and that will break up existing marriages. Long run, people will be with people who they are more compatible with (because of more shared interests, discovered through stuff like facebook, match.com, eharmony, etc.), and thus won't divorce as often.
Short run: Spouses will see they have more options.
Long run: Spouses will have made choices from a wider range of options.
This fits with the Nobel winning work of Shapely and Gale. A major weak point in this perspective is that it assumes human mate preferences are static and that aggregate preferences suffice.
In the real world, "things change" (we've all given or been given that speech). Also in the real world, the heterogeneity of mate preferences is important. A spouse's weakness in a particular area may detract from long term sustainability despite an aparrent short term higher aggregate value.
A major drawback with Match and other online dating sites is that they get courtship out of order by starting with higher brain compatibility.
In the short run, commenters were correct that Facebook, Match, and others will expand the range of outside options for the already-attached (if you ask around, you'll probably meet at least one person for whom the internet contributed to divorce). In the long run, though, websites like Facebook, eHarmony, and Match.com will lead to better matches and, I would expect, less divorce.
There were also good answers to "why won't this work?" regarding my billion-dollar proposal. I'll post on that later, but the basic answer is pretty much the universal Austrian/public choice economist's answer: "information and incentives."