Recruitment through individual contact is really helpfull in two situations
Situation 1: Your team is really small or you do not have a team at this moment. Big communication and social network campaigns would be way to (work-) expensive for you. That´s why you should use individual contacts.
Situation 2: As part of your network marketing strategy. Give reward people for acquiring new members and show them, how they use individual contacts properly.

How to use individual contact
Where you find new community members:
Contacts in first degree
At your former school - maybe you had even lessons with them?
At your (former) university - maybe you know some students which learn to teach this topic
Friends or relatives which (learn to) teach this topic
Contacts second degree
You know someone, who knows someone…
Contacts X degree (actually they are no contacts, but you make them to contacts, when you acquire them)
teacher conferences
Student meetings/ student parties
Private teaching
Open days at shools
At the faculties of schools
Which channels you should use to contact them
Talk to them face-to-face That is the best idea to recruit new members. The only time you should NOT use it, is if you are not able to speak personally to them.
Call them If you can not speak with them face to face, you should at least call them
Write an E-Mail / Facebook / Letter. That is an really impersonal way. You should avoid it. Your request could appear like spam. Serlo is no spam!
How you should acquire them (the 3 magic steps)
I. Ask them for feedback
Talk with them about Serlo (see elevator pitch)
Ask them to use the Website and give feedback to you e.g. ask teacher: "Which topics do you teach next month? Maybe we could help to improve them till next month"
Write them an E-Mail to keep the whole thing running (thank them for their offer to help you, summarize your conversation, show them the link )
II. After they gave you feedback ask them to write Feedback on the website
They can reister on the website and write their Feedback directly there.
Ask them to be really detailed and give concrete ideas for improvement.