If you think you can manage the risk and the spread is ultimately worth it to you, there's nothing stopping you.
I know in the U.S., in CA specifically, you need to have $85,000 annual income or net worth over some threshold to be able to loan more than $2,500 via peer-to-peer loan investing. I've had an account at prosper since around 2010, it does pretty well. It's made my taxes a bit more complicated each year, but it's been profitable for me. I wouldn't lever up on it though.
My Prosper Experience
My experience with Prosper has generally been positive. My real motivation for starting the account was generating a dataset that I could analyze, here's some of that analysis.
I started the account by trickling $100 /month in to buy four $25 loans. Any payments received from these loans were used to buy more $25 loans. I've kept my risk to an average of about A- (AA, A, B, C, D, E, HR are the grades); though the interest rates have reduced over time. At this point, I have a few hundred loans outstanding in various stages of completion. In calendar 2015 I had a monthly average of 0.75% of my loans charged-off and about 3% of loans at some stage of delinquency.
I receive about 5.5% of my principle value in receipts on average each month, including loan pay-offs and charge-offs. Interest and other non-principle payments comprise just shy of 20% of my monthly receipts. Prosper's 1% maintenance fee translates to about 8% of my monthly non-principle receipts. It's all a pretty fine line, it wouldn't take many defaults to turn my annual return negative; though in 5 years it hasn't happened yet. Considering only monthly charge-offs against monthly non-principle receipts I had two net negative months in 2015. I made about a net 4.5% annual return on my average monthly outstanding principle for calendar 2015. When I log in to my Prosper account it claims my return is closer to 7.5% (I'm not sure how that number is calculated).
The key is diversifying your risk just like a bank would. I don't know how the other services function at a nuts and bolts level. With prosper I choose which loans to fund which means I determine my risk level. I assume the other services function similarly.
Regarding collection of charged-off loans. I don't know how much real effort is expended by Prosper. I've had a few notes sold for about 10% of the outstanding balance; and I don't know who they were sold to. Comically, I have loan that's made more payments in collections than when it was in good standing.
There is definitely more to this than handing over $15,000 and receiving 6% on it.