Don't be that random guy telling her she's hot

Your opening line should never include anything regarding her appearance. Aside from being rude and completely unoriginal, it's redundant. Most women will assume that if you're taking the time to message them (or @ reply them, or initiate any other sort of unsolicited contact) you find them attractive.

Because, really, how often does a stranger get in touch with a woman because he thinks she would make a cool, new friend? If your first point of contact is a cheesy compliment, ladies will assume you only got in touch because you think their vagina would make a cool, new friend. 

That said, your main goal should be to show interest without being desperate, pushy, or creepy. Doling out compliments like "Shawty U is fine" or "I just had to tell you how sexy you are" have never—in the history of the Internet—been a successful route to capturing a woman's attention. This approach only incites boredom or anger. Sorry, dudes. You can (and have to!) do better.