I can't say about my old nicknaming habits, but recently I seem to have developed an attachment to names that start with Se and will have handles following that pattern. My current favorite handle is "Sempiternal" but may also include any of the following in combination: Seraphim, Sedulous, Sequester, Sententious, Secern, Secular, Sepulcher, Servile, and etc. (note that all of those words are real and have interesting meanings in the dictionary)
Now, I like to use the anonymity of the internet to play the part of someone I'm not, and not to reveal my true personality. While it's true that the real personality comes out at times, this often only occurs when my identity has already been disclosed. So, for example, here on this site I have no problem being myself (ish) since you all know who I am anyway (also the reason for the obvious username).
At other times, it's fun to pretend to be someone you're not. Since no one can really tell anyway, learning to play the part and decieve people becomes one large game which is always amusing to play. After all, the persona is pretty fake. Of course, there are the rare cases where you meet someone this way who you actually want to be friends with, but odds are that they'd see through your facade first or else not even bother getting to know you.
While it is more comfortable to speak to the opposite gender while using an alias, it's not very fulfilling and doesn't feel like I can accomplish anything that way. Maybe for initial conversations, but real life interactions are better suited for creating and maintaining... "relationships." Not that I would know, yes?
Now, about being different online and offline... I can easily say that I'm more composed online. Since responding online gives me more time to think through my answers, it becomes easier to assimilate style and wit into my responses rather than the spur of the moment that comes with conventional conversation. Sadly, sarcasm is not as effective online because tone is basically non-existent. Also, we all praise the existence of the delete button which allows us to change our sentences without awkward stammering.