My own personal opinion, from direct experience, is that a co-founder is ideally someone that you met non-randomly. You have known or worked with them previously, or have a contact in common that can vouch highly for them.

Of course, you could meet a future co-founder on a 'dating' site, but I would still want to get to know that person over a period of time.

How a person deals with the stresses of making business decisions, handling money and investments, and general product management are not something you get an optimal feel for in a short period of time.

I am sure that there are several people who have met and co-founded a business in a short time window and been happy with the outcome. Just like I know people from arranged marriages or who were married a week after they met, and they are quite happy. But not many people would recommend that approach as having a high probability for success.