Hello, How many links are too many? I would say only have three or four links to other hubs or hubbers within your text. The more links you have away from your hub the more confused readers and search engines get. Your hub should be the one being linked to not the one giving away links.
So to the question of backlinks.
Yes Facebook and Twitter are ok for some link traffic, but they are not available to the majority of users of the internet. The best sources of back links are "expert websites. In this case "expert" is nothing more than longstanding well viewed webpages or hubs, usually over three months old.
So if we imagine you write a series of hubs on a subject, Trainspotting, you publish several hubs over three months, the oldest of these gets good viewing figures and then as you publish the latest hub you link from your oldest to point traffic to the latest. The old hub is "expert" and is to the search engines referencing your latest, that is a good point for the new hub. Someone looking for trainspotting hubs sees that you wrote several and moves between them.
You could also set up a trainspotting blog, this then becomes an independent feature on the net. Your blog as it matures links to several of your hubs and also becomes "expert" to search engines. As long as you add new ideas and feature your work on the tightly focused world of trainspotting the search-engines keep seeing a link.
Then maybe you decide to add places to stay as a additional feature, you can use your trainspotting hubs and blog as "expert" features providing your back links as you build newer content. All the time only linking a few pages at a time within you tightly focused network.
So you are building backlinks all the time, rreferencing "expert" sites which you have created.