So, you had your first date and the spark just wasn't there (on your end, at least). And now you're in a tricky predicament where you might want to thank your person for an okay night, but prefer not to see them ever again. First of all, you don't owe an explanation to anyone you meet on an app after a first date. If the person was rude then don't even bother following up. But if they were pleasant, then it's not necessary to be savage about it. A simple, "It was nice meeting you. All the best with *insert reference from date here*. 🙂" would work perfectly.