Facebook has 3 types of group privacy:
With those types of privacy there are some advantages, but also some obstacles for your advertising.
When you are searching for groups for advertising, this is one of the important thing you should pay attention.
Public groups are visible for every member of Facebook. This type of privacy is the best for advertising because post is visible to all members of Facebook not just for members of group. Visitors can not post to those groups if they are not member of it but they can see posts and they can see group and their members.
Closed groups are little more private then Public groups. Posts are not opened for all members on Facebook but only for members of group. Other visitors can see the group and members of it, but not their posts. So if you want to post your ad in this kind of group you need to have product which will be interesting to members of that group, other way you are just loosing your time posting in it. This kind of groups with less then 5000 members are not valuable.
Secret groups are most private groups on Facebook. If you are not member of it you can not see it. In order that you can see members and posts from Secret group you need to be a member of that group. Also you can not send request for joining, only invitation from members from that group. This kind of groups are not recommended for advertising at all, because your post will not be seen on any circumstances for members who didn’t get invitation for that group.
The best groups for advertising has minimum 2k-3k members, and are public. Groups with 100k+ members are attractive too, but you will have more results with groups with 10k members because of frequency of posting to those groups.
On one account you don’t need more then 400 groups carefully selected, however it is good that you have about 800 to 1k groups on your profile, and Facebook maximum is 5000.
Good luck in your advertising and do not forget to purchase this awesome tool which will help you in advertising on Facebook groups and pages and on other Social Networks: twitter, linkedin, tumblr, wordpress, instagram.