There are certain aspects of my personality that are different between the online world and real life, but those are mostly down to the differences in the medium - online written world vs. physical real world. There are certain things like body language, facial expressions, voice tone and inflection, etc. that don't translate very well into the virtual world, or at all, so it may paint a slightly different picture of what I'm like in person to someone that hasn't met me in real life. I think to a lot of people I talk to, I may seem more interesting online than in person. That's also down to differences in the mediums.
I actually had a real world relationship that went to hell because of too much time spent communicating online and not enough in the real world (lack of opportunities to get together). The differences between the two personalities due to the differences in the mediums drove her away. If it had been 90% real world and 10% online vs the other way around, things would be veeeeeeeery different right now.