Not saying a 5B valuation makes sense, but I think Tinder is employing the thin edge of the wedge strategy [1] (like Instagram did with photos) and is using their huge success with dating to become a more generic social network.

They've already been public with vague plans to move beyond dating. Also, their new verified accounts for celebrities make a lot more sense with a generic social network than with a dating app.

[1] http://cdixon.org/2010/12/27/the-thin-edge-of-the-wedge-stra...