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Please clarify - she's posted her profile to indicate that she is single?
You want to address this and really there is no way to do it than be straight forward. You can ask her open ended questions (if she gets defensive, that is her choice) You may even mention to her that someone had told you that they'd seen her profile and were wondering if you are still together.
It may not be that she's trying to appear available (if there is no option to check dating on the site) she may have just checked singe and not thought about it.
You've been with her for 2.5 years and should be able to talk to her openly. You may even ask her to add you to her facebook list (ask her if she's checked single on her profile what does she think you should indicate on yours)