When should I text him?

Maia

I met this guy two hours ago on the train. Something funny happened and we started to talk about it. At the end I asked if he has Facebook or something. Then he said, that he does have WhatsApp and asked me if he could write it in my phone.

What do I do now? We were smiling at each other the whole time before talking. He seemed like a sweet guy. Now that I have his number, when do you think that I should text him? Tonight or tomorrow or?