Quick question - how many new back links per day to do you think is a good rule of thumb - through social bookmarking, forums, etc. ?
3 - 5 / day ?
Look to obtain links naturally. Obtaining links artificially via discussion forum threads, and/or signatures is a waste of time. Whether your getting 3-5 or 100 links per day, the SE's are a lot smarter than they were 2-3 years ago. Forum links hold little to no weight when it comes to SE ranking - so you would most likely be spinning your wheels.
Most social networks nowadays are 'no follow' (at least the good one's are) - so if you are gonna get involved, go into it for the branding, reach, and potential traffic.