Limerance. I must look that one up.
Interestingly, in terms of your friendship, you mention hanging around during lunch, and chatting on IM.
Those things I never think of as friendship things. I see those very much as acquaintance things. It can be the done thing at work, add a few people from work to your Facebook, have a few chats from time to time. But bear in mind, the world is getting busier. People are getting busier. They might have less time for Facebook which is why the chat is drying up there. Perhaps they're forging other relationships at work. I really wouldn't think too much about acquaintance type friends.
Did you ever go to any pubs, or bars, with them during a lunch break, or after work? Did you or they invite each other out for their birthdays? Or just any other social gathering outside of work and away from Facebook? I have a very strict line when it comes to acquaintance vs friend. If I don't know you outside of the environment where we're thrust together and forced to get on, in other words, if we never go to each others houses or go out for a drink, a meal, even get together and indulge in our common interest, anything, you're not a friend. Why it should just me or you trying, I don't know.
Based on this newer information, I would be inclined to say keep it cool and maybe try inviting them to your next birthday bash. Or say you both like, I don't know, sky diving... whatever the common activity is, why not suggest doing it instead of talking about? (Im learning a lot here myself

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But bear in mind, if they say no, don't take it personally, it might not be about you, they may have other plans, or they might not like the type of environment you've suggested. Sorry again if I've mis understood what you're saying.