Personal statement plagiarism
This topic has been lingering as a "grey" area for a while. There are various aspects of plagiarism that might be exposed in one’s personal statement ranging from having a statement written by a paid service to a paragraph or complete statements "borrowed" from either publicly available statements or from the previously submitted ones, such as your friends who applied year earlier.
A study performed by the Brigham and Women’s Hospital In Boston, Massachusetts published in MedScape article “Level of Plagiarism in Residency Application Essays Worrisome”  revealed that 13.7% of Personal Statements submitted by IMGs to IM, Anesthesiology, and Surgery programs at the hospital were plagiarized.
Is it OK to ask a friend or a colleague to review my statement and may be make some minor edits you may ask. Here is a quote from the Essay written by Ariel Forrester Cole, M.D. in a Society of Teachers in Family Medicine Journal 
Program directors and other faculty reviewing an application use personal statements for various purposes. Many use them as a guide to the applicant’s proficiency in English and motivation for pursuing the residency or fellowship position. The possibility that a personal statement was not written by the applicant calls into question the usefulness of this practice. It is common practice to ask a colleague to review an important writing sample, whether a personal statement for application to a fellowship or an article for submission to a journal. So long as the writing is done by the author, and reviewers, whether paid or unpaid, make only minor revisions, most would argue that plagiarism has not occurred. However, when the writing is so heavily edited as to no longer represent the abilities and ideas of the author, plagiarism has occurred.
The point being made is very precise and clear – if you did not write most of your statement yourself, you may be accused of plagiarism. It is OK to have your statement “polished”. But it is not OK to have it written from scratch or revised to the extent that your writing abilities cannot be judged anymore.
As in any crime and punishment cycle, it is also important to know three major aspects of a "crime":
1. Where is a line between "good" and "bad"
2. How likely it is to have the "bad" revealed
3. What is the punishment
Let’s review these questions one by one.
Where is a line between "good" and "bad"
Dr. Cole’s quote provides a concise answer:
"So long as the writing is done by the author, and reviewers, whether paid or unpaid, make only minor revisions, most would argue that plagiarism has not occurred. However, when the writing is so heavily edited as to no longer represent the abilities and ideas of the author, plagiarism has occurred."
How likely it is to have the "bad" revealed
Geriatrics fellowship program reviewer described in the quoted article happened to notice similarities in two submitted personal statements. Well, fellowship gets 20-30 applications while a typical Family Medicine program gets over 1,000. It is very unlikely that similarities will get noticed while reviewing that many applications, you say. I say, not really. Before reviewing personal statements, a program narrows down selection of applicants to 150-200. Then they split review between several people, so that each reviewer gets to review 30-50 applications. If your and somebody else’s statements have “borrowed” parts, this is going to get noticed.
Plagiarism will be caught with close to 100% probability if a program uses TurnItIn service. While the service itself is somewhat controversial, it detects plagiarism extremely well even if parts of the essay are being “rewritten” by rearranging words or using synonyms. At this point it is unknown what percentage of programs is using the service but the service is being marketed directly to the postgraduate admissions. A few words about TurnItIn, so that you do not underestimate its power. The company keeps a database of everything ever published on the Internet. The controversy comes from storing documents used to check for plagiarism and using them later to check new submissions. As a result, the service can end up having all personal statements ever submitted without statement writers’ consent.
At this point it is unknown how many programs are actually using TurnItIn. There are some rumors that ERAS may incorporate the checks into its system, so the statements get flagged automatically. But we weren’t able to confirm that.
Regardless of whether or not the service is integrated into ERAS, having your statement marked for plagiarism may carry heavy consequences and it is in your best interests to have it written yourself. Light polishing is OK, but heavy editing is not.
And finally, even if everything goes “right” and a statement that you did not write gets accepted and you get an interview invitation, you are in a hot water. If your speech and writing skills are not equal to what is in your personal statement, this will become apparent to all interviewers and your professional skills will be judged appropriately. So, the picture should be pretty clear, it is extremely likely that plagiarism will get revealed.
What is the punishment
Your application will be marked as “irregular behavior” and passed to ERAS for further actions. Regardless of how harsh are the actions, having an “irregular behavior” mark on your application will be detrimental to getting interviews. Application may be also banned from use of ERAS service next year, which also translates to no interviews.
Bottom line – you definitely do not want this to happen. Write your statements on your own. Use friends and colleagues to proof-read them. But do not use passages from the Internet or your friends’ statements.
What about paid services for writing personal statement, you may ask. I want you go back to the Dr. Cole’s quote. As long as they make only minor corrections, you should be fine. If there is anything more than that, you are in a hot water. I also want to refer to the last point made in the “How likely it is to have the bad revealed”. A personal statement that does not match one’s writing and conversational skills is a clear indication of use of services.