The acquisition is part of LBi's plan to expand its service in the US, which it outlined in its strategic roadmap launched earlier this year.
During 2007, Special Ops, which has 64 staff, had gross billings of $22.3m and net sales of $10.1m.
LBi will pay between $20m and $45m, depending on an earn-out arrangement based on future performance.
Luke Taylor, LBi CEO, said: "Special Ops have, in particular, unique skills in innovative social network marketing and online PR. We plan to leverage this capability across our global footprint on the back of strong client demand for these specialist services."