You can monitor people on Twitter complaining about the problem you solve, @reply them to check at your site, and then chat on oLark/SnapEngage to help convert them into users (or just do customer development).
I put in hashtag monitors into hootsuite for #html, #css and #wysiwyg (I've found using hashtags makes people much less annoyed at receiving a reply out of the blue) and then replied to people with a personal message who were chatting about them.
Because I could do it on my phone (ie. on the train, waiting in line) it was basically "free" time.
I wrote maybe 500 messages, got about 120 people to the page and about 57 of them put their email address in, all within 3 days of launching.