Some people like to use phrases like 'Blessed Be', and 'So Mote it Be', when talking to other pagans online. However, don't be surprised if someone asks you not to, or might possibly be offended by your doing so.
These phrases are ritual language. That is to say, they are phrases used within the structure of Wiccan ritual for very specific purposes, and outside of that context they may be meaningless, or incorrectly applied. Some Wiccans feel that using them out of context cheapens or degrades their sacredness. Others may not feel as strongly.
'Blessed Be' is a very specific blessing. It's not a greeting or a parting, and by using it as such you're showing you don't know what it properly means, or that you don't value its' importance to Wiccans.
If you wouldn't sign off your casual posts online with "Body and Blood of Christ", then you shouldn't do the same with "Blessed Be". It's exactly the same kind of sacred language, and it doesn't easily apply to things outside its' proper context.
It happens all the time. There will always be people who pepper their speech or writing with these phrases are trying hard -too hard, really- to fit into a subculture they feel insecure about. We all know people who overuse the jargon for certain groups or interests in order to look experienced or knowledgeable...but quite often it's too much the other way, and you end up looking like a noob.
There's nothing wrong with just talking to other pagans, or specifically Wiccans, like you would anyone else. "Hi" and "TTYL" work just fine, instead of "Merry Meet" and "Merry Part", and they're not going to upset anyone or make you look over-enthusiastic. Save the formal ritual language for ritual.