Is it slanderous for a person to lable another "Mentally unstable and with serious issues" online?

Somebody I believed was a friend of mine, wrote online, on my Facebook page, that I am a "mentally unstable" individual, with serious issues. He wrote it where everyone on my Facebook page(family, friends) could see this, and since, many have questioned me about it, and are acting differently towards me than before. In response to a Facebook heading I wrote "�1400 tax rebate at Christmas, yeehah!" my so called friend, who I hadn't seen for two years, wrote back in a y manner "You dope, you should check your tax first". I responded to this by saying that someone like him shouldn't call anyone a dope, and it's ridiculous that when you haven't seen someone for two years, the first thing you say to them is that. In response, he called me a mentally unstable individual, saying "we" meaning other friends, all think this too, although they have no grounds for it as I'm a placid person. I just want to know is this what they call "Internet Slander" and can anything be done about it?