“Probably” has several different sound shapes – that is, it can pronounced in different ways. You can often hear the full pronunciation, with three syllables:
I should probably… Probably many things… Probably the only place
But that sequence of two [b] sounds can be a little awkward when you speak quickly. So it’s very common to simplify it to [pɹɑ́bli]:
I probably would… probably be the best solution… probably heard me say this
In quicker speech, both ‘b’s may disappear:
I think they’ll probably come tomorrow.
Some say that the spelling “prolly” is becoming more common in text messages, but it’s not just a recent innovation. John McIntyre of the Baltimore Sun considers the word in more depth, finding references to this pronunciation as far back as 1922.