One of the great features of online linking with other professionals is the more casual way that references--or, in this case, endorsements--are sought.
As someone who has previously been asked by students I have only known for a short time for a reference I find this looser way of expressing good will much more comfortable and authentic to the situation.
Previously when students sought serious recommendations I often politely refused. My answer sounded like this: I have only known you for sixteen weeks and we have only met twice a week. I did most of the talking.
When I recommend someone for a job I need to know more--say more.
But that's not the case on web forums like Linkedin. where a question similiar to "Can you help me out?" means: Is there something good you can say about me?
To loosely quote Will Rogers: I've never met a man (or student) I didn't like. Actually Rogers said something like that after a meaningful trip to Russia when he said that once you get to know someone, it's not hard to like that person.
You can't really know everything you need to know about someone to recommend them unequivocally for employment. But you can have this genial good will and that also matters. Prospective employers have their own way of assessing skill sets, but they also want to know if this applicant can get along with other people.
Now, when someone I'm linked with (usually a student)asks, "Can you help me out?" I just hit the reply button and write something true about how I feel.
It may not be the most professional way of recommending someone, but it's true--and that's pretty powerful.