As with 'real life' scenarios, you need to try to work out how the other person might interpret your contact with them. That's especially true if the other person might see your posts as potentially worrying. For example, if you've just noticed an attractive young woman posting swimsuit pictures of herself on Facebook, diving straight in with the type of questions suggested by Jennykenny might be unwise. (However asking the same type of questions of someone on a website used by people deliberately seeking new friendships might be seen as perfectly normal).
Seeking a common interest might be a better starting point. For example, if I was drinking in a bar and someone came up to me and immediately started asking me about my job and where I was from, I might feel uneasy. However if they started off by saying "I see that you're a real ale drinker. What can you recommend in this pub?", I'd regard that as perfectly normal. (A few pints later we'd probably both know the other's full life history anyway!).
So something like "I loved the pictures of your cat that you posted ; I used to have one just as daft as that myself" might be a good opening gambit.