Well, at least you can say that the 20k € was spent to learn that signups from Facebook have a near-zero retention rate. (That's not meant to be a flippant comment.)
A lot of successful websites seem to know (or learned) which market channels bring in the right customers.
For Stackoverflow.com: most traffic comes via referrals from google.com searches -- but not facebook
For Craftsy.com: it's referrals from Facebook -- but not google search queries
For a project management tool like yours, neither Google Adwords(1) nor Facebook newsfeed ads will work. Maybe a strategy such as hosting for free a high-profile open-source project so your brand gains mindshare. Then it may attract non-open-source paying customers.
That subsidy strategy for non-profit customers is not foolproof (as it didn't seem to help BitKeeper) but it did seem to work for Github.
(1) I suppose you could try running an AdWords campaign on keywords such "project management" and related competitor keywords such as "Jira" and "Microsoft Project" -- but I doubt you'd get quality clicks from it