First, you should use emoticons, as they are called, as liberally as possible, unless you are politically conservative, in which case you should use them frequently.
Each has a specific effect as supplemental (or as a replacement for) punctuation.
Let's compare and contrast, shall we?
I robbed a bank yesterday.The first is a frank admission of guilt, or a matter-of-fact description of one's accomplishments. Devoid of context, it is bland and unassuming. The second positively beams with pride.
I robbed a bank yesterday :)
I robbed a bank yesterday ;)
The last, however, adds a pinch of flirtatiousness: If you're lucky, you just might end up a little richer. So, while :) is ambiguous, albeit friendly, ;) adds a hint of sauce. If you're going to employ it, be aware of this critical fact.
Also, I should note that for good reason, in academic writing, the use of emoticons is generally frowned upon. >:(
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