Many great answers here already.
If I was stepping back from the immediate question, I would suggest examining Facebook's rationale for any new product it launches.
What is the purpose? Why does it continue launching and acquiring?
Fundamentally, you can reframe Facebook's strategy to be around maintenance vs. growth, given its position on the s-curve of mass adoption. It is clear from Facebook's history of recent acquisitions, Zuckerberg is very well aware of the innovator's dilemma, and attempting to disrupt the core business and diversify at the same time.
For example, with WhatsApp (a closed P2P network), or Instagram (a separate photo sharing platform i.e. usurping what was Facebook's original raison d'etre).
What is the greatest existential threat to Facebook maintaining its scale and dominance? Losing relevance.
Any network can only remain relevant so long as it delivers value to all members on an ongoing basis. A loss of relevance occurs through 1000 cuts, not 1.
This I would imagine would be a core concern in the minds of FB's board, major shareholders and the ExecCo.
One form of relevance is maintaining a moat from network effects, but Facebook's biggest issue at present is churn and the distribution of its network skewing away from the early adoptors and cultural influencers.
Will the launch of a dating product (albeit a generic one, just on Facebook), mean it increases engagement with a highly valuable demographic for advertisers and reduces churn due to lock-up on the network? Possibly.
Are there likely to be strong data crossovers from a richer social graph if combined with dating patterns? Highly likely.
Will 18-34yo adopt Facebook dating when they've been slowly transitioning off the platform as its been unbundled over the past 3-5 years? Time will tell.