I am a Private Investigator in Lafayette, La and working on this case independently and would like to point a few things out.
While there are many good theories in this thread, it is important to focus on the facts and only the facts to form theories as well as look at the most simple explanations for her disappearance.
Scenario 1: Hit and run. While possible, highly unlikely. The sound of a wreck is unmistakable and loud. Also, someone who accidentally hits someone is more likely to speed off rather than further incriminate themselves by getting out, risk being seen, hide a person and a bike and leave more evidence. So possible, but unlikely
Scenario 2: Abducted by stranger. First, this doesnt happen all that often around here and its normally with children. Most acts of random violence are typically robberies. So its quite possible someone saw her alone and thought to steal her bag and bike. However this is where it becomes highly illogical. Someone just commiting a robbery, again like a hit and run, would not want to further incriminate themselves. These crimes are usually quick. Knocked off bike, stole bag, sped off. Simple. Even if a fight ensued and god forbid she died, the person would most likely get away as fast as possible. There would be evidence of a struggle somewhere and i think she or her belongings would be found by now. Taking a person AND a bike withought leaving a trace is just too difficult and too much trouble for some random crime.
This leaves us with the most likeliest of scenarios. She stopped to visit someone or someone she knew saw her and offered her a ride home. This is where something went wrong and whatever happened happened. This would account for why theres no one who saw or heard anything, why nothing of hers has been found, and the general lack of evidence. This is the most likely situation and is the angle that should be focused on.