There alot of things involved.
Mostly, I'd say that everyone has a life. This site is not what most would call a 'priority'. Sure, when you think you've started a friendship with someone and are interested in that continuing, you must put up an effort. But even good people are stuck with 24 hours in a day.
Be patient. Be understanding.
Not replying to an unsolicited email is not bad manners. Think of it this way ... if the person responds, most likely you've made a friend. To me, that is enough.
Just keep being you - no matter what someone else does/thinks/says.

So its not just me. I all most gave up emailing anyone never get one back. I even say thanks for checking me out when someone looks at my profile. All most never get a reply back.

Emailing is a way to begin to get to know a person and select out those that you don't connect with on a more intimate basis....Sometimes after a few conversations the interest level may dwindle....Compatibility has many factors, but mostly shared interests and experiences....So if it ain't happening for one of you that's why the disappearing act...I've always found if the interest is there, than live communication should be the next step....