Those are just signatures.
Some people are using abbreviations in their signatures.
Some people are even lucky that their signatures are readable.
To be honest I can't even read William Shakespeare's signatures.
The signatures and the correct spellings are different things.
The correct spelling of his name is still "William Shakespeare".
The definition of Urban Dictionary about this is still correct.
No matter, if you would use it as a source or not.
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Who in the world trusts Urban Dictionary?
This is not just about Urban Dictionary.
But - since it's crowdsourced -, a lot of people, if you ask.
"...Urban Dictionary averages 72 million impressions and 18 million unique readers."
"Anyone with either a Facebook or Gmail account can make a submission to the dictionary, and it has been stated that entries are reviewed by 20,000 volunteer editors."
Back to the writing of the name of Shakespeare:
William Shakespeare used the "Shakespeare" form in the first editions of the Venus and Adonis (1593), which is probably Shakespeare's first publication and he also used this form in The Rape of Lucrece (1594), which is currently considered as his second publication. This ("Shakespeare") form was the most common published form in Shakespeare's lifetime.
The form of "Shakespear" was used in the past (in the 18th century), but nowadays it is considered as a mis-spelling by most of the people.
I have got these quotes and the previously mentioned informations from the Wikipedia, which is also used by a lot of people (much more than Urban Dictionary).
Sources:
Spelling of Shakespeare's name - Wikipedia.
Urban Dictionary - Wikipedia.