Unless the picture is casting some illusions, that's way too big to be serviceberries - even the big ones they grow in orchards are only half that size. ServiceBerries are also known as JuneBerries, and around here they just finished producing for the year.
If I had to guess, it
might be a plum - the oval-shape reminds me of them anyways. Again, that's purely a guess. If you could dissect one it would be easier to identify
