What I like to do when I get a number close is to tell her what I intend to do.
When you get her number and ending the interaction, say something like "Great, I'll text/call you tomorrow" and stick to what you said you would do.
Doing so gives her that feeling of sincerity of your actions - She knows to expect you contacting her and that you're a man of your word.
The exchange of Facebook info is fine if she is comfortable or even suggested that - So all in all it sounds like a solid exchange and interaction.
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i know some girls do the "wait 2 days" rule the same aas some guys used to do, so maybe she was able to see your phone number without opening the message and just haven't decided to text you yet.....i would wait another day or 2 before i get worried
This is a valid point and is quite normal for some to do this - I will confess, I even used to follow this rule purely because a friend of mine once said I should wait a day or two. This was before I first discovered pickup.
You want to be seen as a guy who is mixing things up a little, you don't want to be like every other guy who might also follow this '2 day rule' - In reality you could be on a date or even f closed her in that length of time.
Going back to the top of my post, this is a good way to combat this unwritten 2 day rule, its used by people to seem like they are not 'needy' or desperate, but personally I think you lose a lot of that initial rapport built during the interaction and can harm the chances of actually seeing this person again.
Edit: Spelling/Grammar