"Crochet is period." 

Tatting -- no, it's not period for the 18th century, despite claims by a few badly researched books on tatting. Here's an article on the history of tatting.  Netting, on the other hand, is period; it is a different technique that uses a shuttle similar to a tatting shuttle.  So if you see a lady with a shuttle in her hand, she's netting.

Crochet -- Unfortunately, no, not for the 18th century, though if you're a 19th century reenactor you might be able to use it (depending on which decade you do).  Here's an article from a crocheting site on the history of crochet.

Just because neither of these techniques is period for the 18th century doesn't mean you can't enjoy them for modern use.  Heck, lace-knitting isn't period either, but I'm knitting a lovely lace shawl that I intend to wear with modern clothing.  Who says all your hobbies HAVE to be period?