While the approach that you outlined does in fact work to widen one’s social network, it is not dating. It is using dating sites as another venue for networking. The average person joins a dating site in search of a romantic interest.
The behavior that drives us couple and the traits that we seek were selected under pressure back when we were hunter-gatherers. Women are driven to couple due to the primal need for safety and security (it is the reason why women prefer tall, broad shouldered men). A relationship with the right man fills this need. Guys are driven to couple by a response known as the primal urge to pursue. Both primal responses require age and experience to be able to moderate.
A true friends-first approach is about patience. It is about respect. It is about taking the time to get to know each other and allow romance to blossom. The journey is important as the destination. Too many young people demand instant gratification. The old adage about buying a cow when the milk is free applies to friends-first. Women have made access to sex cheap; therefore, men have no reason to invest in a committed relationship. If a woman wants a man to love and protect her, she needs to make him feel like he is special. If man wants a woman who he can respect enough to marry, he needs to stop sleeping with every woman who says, “yes.” Sex and romance are not a game.