Origin and use of the abbreviation 'u' for ‘you’

According to the Oxford English Dictionary, it has been used in IOU since 1795:

An I.O.U. is admissible evidence of a debt without a stamp.
_{Espinasse's Rep. (margin note)}

As for standalone examples, the first one given by the OED is from 1840:

There was no one but the plaintiff to whom the ‘U’ in the document [an IOU] could be applied. _{Manning and Granger's Reports}

The next oldest example given is from 1862, in a letter from Thomas Hardy to his sister:

I wish you wd tell me how u.r. when u. write.

After that, the OED lists a source which mentions it in an advertising context:

This is another device which appears to be growing in popularity. Where possible a capital letter is often substituted for writing out of a syllable.

Call Tel-U-Where for our nearest distributor... Fits-U Eyeglasses. U All Kno After Dinner Mints. Uneeda Biscuit.
_{Dialect Notes}